Robot Analysis The Mechanics Of Serial And Parallel Manipulators Pdf

Peter Corke, Robotics, Vision and Control, 2013 — Online via SprigerLink; Richard Szeliski, Computer Vision: Algorithms and Applications, 2011 — Online via SprigerLink; Lung-Wen Tsai, Robot Analysis: The Mechanics of Serial and Parallel Manipulators, 1999 — Available at UQ Library; Oussama Khatib and Krasimir.

Robotics II Course Page Quick Access: ► ► ► ► ► ► ► ► ► Related pages: ► ► ► ► ● ● ● ● ● ● ● ● Digital Control ● Theory of linear systems ● Advanced dynamics ● Advanced industrial control ● ● Course Description Course Name: Robotics II (Parallel Robotics) Course No. EE - 43122 Professor: Semester: Spring 86 Room and Time: Sat 8:00-10:00 Wed: 10:00-12:00 Room 204 Office Hours: Sat-Mon: 16:00:17:00 In this course the kinematics, dynamics analysis and control of parallel manipulators are studied in detail. A review of kinematics analysis of serial manipulators, which is detailed in the course is presented here with an addition topic of screw and wrench representations.

The main emphasis of the course is the mechanics of complex structured robots such as parallel manipulators. Comprehensive kinematics and dynamic analysis of parallel manipulators is presented, and the control topologies for these robots are described. In a comprehensive design project the students learn how to analyze the kinematics and dynamics of a planar parallel manipulator. The tentative course contents are as following. Time: Teaching Contents Week 1 Introduction: Robotics at a glance, kinematic chains, Grubler criterion, loop mobility criterion, robot classifications, Week 2 Introduction: Description of position and orientation, screw-axis representation, Euler angle representations. Week 3 Kinematics: Review on position analysis of serial manipulators, Denavit Hartenberg convention, loop closure equations, kinematic analysis or 3R manipulator, successive screws, Elbow robot.

Week 4 Kinematics: Kinematics analysis of parallel manipulators, vector loop equations, 3RRR manipulator, spatial orientation manipulator and Stewart Gough manipulator. Week 5 Jacobian: Angular and linear velocity, Jacobian matrices, Singularity conditions, conventional Jacobians, 3RRR manipulator, spatial orientation manipulator and Stewart Gough manipulator, Screw-based Jacobians. Week 6 Stiffness: Force-moment relations, principle of virtual work, 3RRR manipulator, stiffness analysis of parallel manipulators, stiffness analysis of Stewart-Gough platform.

Week 7 Midterm Exam Week 8 Dynamics: Dynamics analysis of parallel manipulators, Newton-Euler formulation, dynamic analysis of Stewart-Gough platform. Week 9 Dynamics: dynamics analysis of parallel manipulators, Principle of virtual work, Week 10 Dynamics: Lagrange formulation, dynamic analysis of CKCM Robot, properties of manipulator dynamics. Week 11 Control: Introduction to control of parallel manipulators, position control topologies, Nonlinear Control Background. Week 12 Control: Position control: inverse dynamics control.

Week 13 Control: Robust inverse dynamics control, Force control topologies stiffness control. Week 14 Control: Force control topologies: Direct force control, impedance control. Week 15 Hybrid Control: Force-position control, matrix inclusion method, hybrid force-motion control topology. 1 Lung-Wen Tsai, “Robot analysis: the mechanics of serial and parallel manipulators”, New York, Wiley, 1999.

Hutchinson, M. Vidyasagar, “Robot Modeling and Control”, New York, Wiley, 2006. Sciavicco, B. Siciliano, “Modelling and Control of Robot Manipulators”, Springer Verlag, 2nd ed. Merlet, “Parallel robots”, Boston, MA: Kluwer Academic Publishers, 2000. 5 Carl Crane, “Screw theory for spatial robot manipulators”, Cambridge, Oxford, 2005.

6 Selected papers.

(1967) The Horta was an example of. Now we are really sailing off into terra incognito. 'Here be dragons' and all that. But if you have starships, you almost have to have aliens (Isaac Asimov's Foundation trilogy being the most notable exception). The 'science' is called, the famous 'science in search of a subject'.

Unfortunately it only offers vague generalities. You can keep up on the, but for now if you want aliens, you are going to have to.

Suggested reading includes, Steve Colgan's blog, by David Darling, by Clifford Pickover and by Stanley Schmidt. Sapient [first use unknown] Sometimes contrasted with `sentient' because even low animals can feel. `sapient' is usually an adjective, `sophont' usually a noun. Sentient [first use unknown, but goes back at least to 1940s] General SF term for an extraterrestrial or alien possessing human-level intelligence (see sophont). Etymologically, and in mainstream English the word means 'feeling' but is rare and now archaic. Sophont [From Poul Anderson's `Polesotechnic League' stories, going back at least to 1963] An evolved biological intelligence.

Implies human-level cognitive and linguistic ability but not necessarily tool use. More specific and etymologically correct than sentient. Still less common than that term, but has been used by multiple writers.

Intelligent races who are not. The term as such is never used for non-intelligent species, however unearthly, though in these may be called Alien Life Forms. Nor is it used for Earth Humans who must register with the immigration service. In general, Aliens fall into two distinct groups, and. 1) Really Aliens are truly unearthly.

Frequently reported species include Energy Beings,, Giant Insectoids (who may also be Hive Entities), and Blobs of Protoplasm. The occasional intelligent bear or radish may also appear, or practically anything else. Except for the Energy Beings, most seem to be hydrocarbon life forms, but methane breathers who thrive at -200 C will sometimes turn up. What they all have in common is that they are Really Alien. Exosemanticists have their work cut out understanding them, and exopsychologists in figuring out what they're all about.

Relations between humans and Really Aliens are necessarily limited, since we have so little in common with them. Only rarely will anybody get to know one on a personal level. With them is sporadic, and even seems less frequent than it used to be in the.

This is partly because it is not clear what we would fight them over, and partly because they may have an alarmingly high, making war with them a dangerously one-sided proposition. Dangerous at least for us.

2) Aliens with Forehead Ridges. Much more common - especially in - than Really Aliens, these are species that look almost exactly like Earth Humans, except for some distinguishing visible feature such as, well, forehead ridges, or odd-shaped ears, or whatever. Sometimes they look rather less like humans, in which case (if friendly) they often resemble large teddy bears. Not only do Aliens with Forehead Ridges mostly look like Earth Humans, they tend to act like Earth Humans as well, or at least one particular (real or speculative) Earth Human culture. A particular race of Aliens with Forehead Ridges may all have a culture like that of medieval Japan, or one based entirely on music, but you will very rarely find more than one culture per species. (The Vulcans and Romulans of Trek fame are a rare exception.) Because of the similarity (or at least comprehensibility) of cultures, Earth Humans can have far more complex and intimate relations with Aliens with Forehead Ridges than with Really Aliens. We can not only communicate, Trade, and fight, but form joint business ventures, cheat each other at cards, and even fall in love.

Indeed, Aliens with Forehead Ridges raise a profound question in evolutionary biology. Convergent evolution might well produce a generally humanoid body plan, just as sharks and dolphins have a similar overall configuration. But Aliens with Forehead Ridges have much more than a general similarity to Earth Humans. They have the same secondary sex traits - as species-specific as you can get. Only their males have much facial hair, and their females often have bodacious figures.

Often, indeed, they are species. This leads to some speculation that they may be of Earth Human descent. (Or else Earth Humans are descended from them, though this raises troublesome questions about chimpanzees.) Perhaps because of these awkward issues, Aliens with Forehead Ridges have become much less common in written (save for media tie-ins) than they were some decades ago. In written SF, the seems increasingly to be inhabited only by Earth Humans. However, Aliens with Forehead Ridges continue to thrive in Hollywood Scifi.

This is for an obvious reason: the audience wants aliens of some sort, and Aliens with Forehead Ridges are the only kind that can be played by members of the Screen Actors' Guild. One of the first ways in which we learn to classify objects is into two groups: 1. Living and 2. In casual encounters with the material universe, we rarely feel any difficulty here, since we usually deal with things that are clearly alive, such as a dog or a rattlesnake; or with things that are clearly nonalive, such as a brick or a typewriter. Nevertheless, the task of defining 'life' is both difficult and subtle; something that at once becomes evident if we stop to think. Consider a caterpillar crawling over a rock. The caterpillar is alive, but the rock is not; as you guess at once, since the caterpillar is moving and the rock is not.

Yet what if the caterpillar were crawling over the trunk of a tree? The trunk isn't moving, yet it is as alive as the caterpillar. Or what if a drop of water were trickling down the trunk of the tree? The water in motion would not be alive, but the motionless tree trunk would be.

It would be expecting much of anyone to guess that an oyster were alive if he came across one (for the first time) with a closed shell. Could a glance at a clump of trees in midwinter, when all are standing leafless, easily distinguish those which are alive and will bear leaves in the spring from those which are dead and will not?

Is it easy to tell a live seed from a dead seed, or either from a grain of sand? For that matter, is it always easy to tell whether a man is merely unconscious or quite dead?

Modern medical advances are making it a matter of importance to decide the moment of actual death, and that is not always easy. Nevertheless, what we call 'life' is sufficiently important to warrant an attempt at a definition. We can begin by listing some of the things that living things can do, and nonliving things cannot do, and see if we end up with a satisfactory distinction for this particular twofold division of the Universe. 1.A living thing shows the capacity for independent motion against a force. A drop of water trickles downward, but only because gravity is pulling at it; it isn't moving 'of its own accord.' A caterpillar, however, can crawl upward against the pull of gravity.

Living things that seem to be motionless overall, nevertheless move in part. An oyster may lie attached to its rock all its adult life, but it can open and close its shell. Furthermore, it sucks water into its organs and strains out food, so that there are parts of itself that move constantly. Plants, too, can move, turning their leaves to the sun, for instance; and there are continuous movements in the substance making it up. 2.A living thing can sense and it can respond adaptively.

That is, it can become aware, somehow, of some alteration in its environment, and will then produce an alteration in itself that will allow it to continue to live as comfortably as possible. To give a simple example, you may see a rock coming toward you and will quickly duck to avoid a collision of the rock with your head. Analogously, plants can sense the presence of light and water and can respond by extending roots toward the water and stems toward the light. Even very primitive life forms, too small to see with the unaided eye, can sense the presence of food or of danger; and can respond in such a way as to increase their chances of meeting the first and of avoiding the second. (The response may not be a successful one; you may not duck quickly enough to avoid the rock—but it is the attempt that counts.) 3. A living thing metabolizes. By this we mean that it can eventually convert material from its environment into its own substance.

The material may not be fit for use to begin with, so it must be broken apart, moistened, or otherwise treated. It may have to be subjected to chemical change so that large and complex chemical units (molecules) are converted into smaller, simpler ones. The simple molecules are then absorbed into the living structure; some are broken down in a process that liberates energy; the rest are built up into the complex com­ ponents of the structure. Anything which is left over, or not usable, is then eliminated. The different phases of this process are sometimes given separate names: ingestion, digestion, absorption, assimilation, and excretion.

A living thing grows. As a result of the metabolic process, it can convert more and more of its environment into itself, becoming larger as a result. A living thing reproduces. It can, by a variety of methods, produce new living things like itself. Any object which possesses all these abilities would seem to be clearly alive; and any object which possesses none of them is clearly nonalive. Yet the situation is not at all clear-cut.

An adult human being no longer grows and many individuals never have children, but we still consider them alive even though they no longer grow and do not reproduce. Well, growth takes place at some time in life and the capacity for reproduction is potentially there. A moth senses a flame and responds, but not adaptively; it flies into the flame and dies. Ah, but the response is ordinarily adaptive, for it is toward the light. The open flame is an exceptional condition. A seed does not move, or seem to sense and respond—yet give it the proper conditions and it will suddenly begin to grow. The germ of life is there, even though dormant.

On the other hand, crystals in solution grow, and new crystals form. A thermostat in a house senses temperature and responds adaptively by preventing that temperature from rising too high or falling too low. Then there is fire, which may be considered as eating its fuel, breaking it down to simpler substances, converting it into its own flaming structure, and eliminating the ash which it can't use. The flame moves constantly and, as we know, it can easily grow and reproduce itself, sometimes with catastrophic results. Yet none of these things are alive.

We must therefore look at the properties of life more deeply, and the key lies in something stated earlier: that a drop of water can only trickle downward in response to gravity, while a caterpillar can move upward against gravity. There are two types of changes: one which involves an increase in a property called by physicists, and one which involves a decrease in that property. Changes that increase entropy take place spontaneously; that is, they will 'just happen by themselves.' Examples are the downhill movement of a rock, the explosion of a mixture of hydrogen and oxygen to form water, the uncoiling of a spring, the rusting of iron. Changes that decrease entropy do not take place spontaneously.

They will occur only through the influx of energy from some source. Thus, a rock can be pushed uphill; water can be separated into hydrogen and oxygen again by an electric current; a spring can be tightened by muscular action, and iron rust can be smelted back to iron, given sufficient heat. (The entropy decrease is more than balanced by the entropy increase in the energy source, but that is beside the point here.) In general, we are usually safe in supposing that any change which is produced against a resisting force, or any change that alters something relatively simple to something relatively complex, or that alters something relatively disorderly to something relatively orderly, decreases entropy, and that none of these changes will take place spontaneously. Yet the actions most characteristic of living things tend to involve a decrease in entropy. Living motion is very often against the pull of gravity and of other resisting forces.

Metabolism, on the whole, tends to build complex molecules out of simple ones. This is all done at the expense of energy drawn from the food or, ultimately, from sunlight, and the total entropy change in the system including food or the sun is an increase. Nevertheless, the local change, involving the living creature directly, is an entropy decrease. Crystal growth, on the other hand, is a purely spontaneous effect, involving entropy increase.

It is no more a sign of life than is the motion of water trickling down a tree trunk. Similarly, all the chemical and physical changes in a fire involve entropy increase. We become safer, then, if we define life as the property displayed by those objects which can—either actually or potentially, either in whole or in part—move, sense, and respond, metabolize, grow, and reproduce in such a way as to decrease its entropy store. Since one sign of decreasing entropy is increasing organization (that is, an increasing number of component parts interrelated in increasingly complex fashion), it is not surprising that living objects generally are more highly organized than their nonliving surroundings.

The substance making up even the most primitive life form is far more variegated and complexly interrelated than the substance making up even the most complicated mineral. What about life forms radically different from ours, based on altogether different kinds of chemistry, living in completely hostile (to us) environments? Could there conceivably be a silicon-based life, in place of our own carbon-based one, on a hot planet like Mercury? Could there be an ammonia-based life, in place of our own water-based one, on a cold planet like Jupiter? We can only speculate. There is absolutely no way to tell at present.

We can wonder, though, whether human astronauts, ex­ploring a completely alien planet, would be sure of recog­nizing life if they found it. What if the structure were so dif­ferent, the properties so bizarre, that they would fail to realize they were facing something sufficiently complex and organized to be called living? For that matter, we may be facing such a necessary broadening of the definition right here on Earth in the near future.

For some time now, men have been building machines that can more and more closely imitate the action of living things. These include not merely objects that can imitate physical manipulations (as when electric eyes see us coming and open a door for us) but also objects that can imitate men's mental activities. We have computers that do more than merely compute; they translate Russian, play chess, and compose music.

Will there come a point when machines will be complex enough and flexible enough to reproduce the properties of life so extensively that it will become necessary to wonder if they are alive? If so, we will have to bow to the facts. We will have to ignore cells and DNA and ask only: What can this thing do? And if it can play the role of life, we will have to call it living. Our main point is that for many modern readers, a violation of the laws of thermodynamics by the author can spoil a story just as effectively as having Abraham Lincoln changing a set of spark plugs in a historical novel.

Therefore, if we travel to Mars in a story, the vehicle must operate either along physical laws we currently think we know, or at least on more or less convincing extrapolations of those laws. Furthermore, when we get there the Martians, not to mention their lapdogs, saddle horses, dinner steaks, and rheumatism, must not strike too jarring a set of notes against the background which author and reader are, it is to be hoped, visualizing together. It is permissible and even desirable to take the reader by surprise with some of these details, of course. However, his reaction to the surprise should be the urge to kick himself for failing to foresee the item, rather than resentment at the author’s ringing in a new theme.

It follows that the “hard” science fiction writer must have at least an informed layman’s grasp of biochemistry and ecology. Even in this narrowed realm, there would seem to be two basic lines of procedure for the storyteller who needs nonhuman characters and other extraterrestrial life forms. The two are not mutually exclusive; they overlap heavily in many ways.

Nevertheless they represent diiferent directions of attack on the problem, one of which is more useful if the basic story is already well set up in the author’s mind, while the other is of more use in creating and developing the story possibilities themselves. In the first case, the qualities of the various life forms have to a considerable extent already been determined; they are demanded by the story events. Excellent recent examples occur in some of Keith Laumer’s “Retief” novels, such as the in Retiefs War and the even more peculiar Lumbagans in Retief's Ransom. In other words, if the savages of Fomalhaut VII are going to kidnap the heroine by air,. If the hero is going to escape from a welded-shut steel safe with the aid of his friend from Regulus IV, the friend must be able either to break or dissolve the steel, or perhaps get into and out of such spaces via the. These are part of the starting situation for the author, who must assume that the creations of his intellect do have the requisite powers. If he is really conscientious (or worries greatly about being laughed at by scientific purists) he will also have in the background an ecological system where these powers are of general use and which contains other creatures whose behavior and abilities fit into the same picture.

Flying must be easier on Fomalhaut VII than on Earth. Perhaps the air is denser, or the gravity weaker, or native muscle more efficient and powerful. Ordinary evolution will have been affected by the fact that flight by larger animals is possible, so there will be a much wider range of large flying organisms than we know on Earth. There will be carnivores, herbivores, and omnivores.

There will be a wide range of attack and defense systems among these beings. In short, there will be more ecological niches available to large flyers, and it may be confidently expected that evolution will fill them. Of course there will be limits, just as on Earth. Vertebrates have been flying for nearly two hundred million years, which for most of the forms involved means about the same number of generations; but we have no supersonic birds on this planet.

Even the insects, which have been flying a good deal longer, haven’t gotten anywhere near Mach 1; the eight-hundred-mile-per-hour deer-bot fly which appeared in the literature during the 1930s was very definitely a mistaken observation. It would seem that our biochemistry can’t handle energy at the rates needed for supersonic flight.

It is the evident existence of these limits which forces the author to assume a difierent set of conditions on the Fomalhaut planet. Similarly, fourth-dimensional extrusion will have to be general on Regulus IV, and the local ecology will reflect the fact. There will be hide-and-seek techniques among predators and prey essentially incomprehensible to human beings, and therefore a tremendous challenge to the imagination and verbal skill of the writer. If fourth-dimensional extrusion is not the answer chosen, then the ability to dissolve iron may have developed—which implies that free iron exists on the planet under circumstances that make the ability to dissolve it a useful one. Or There is, of course, a limit to the time any author can spend working out such details. Even I, a spare-time writer who seldom saddles himself with deadlines, spend some of that spare time writing the story itself.

In any kind of story whatever, a certain amount of the background has to be filled in by the reader’s/listener’s imagination. It is neither possible nor desirable to do everything for him.

In this first line of attack, the time and effort to be spent on detail work are reasonably limited. Even the second line, which is my favored technique, has its limits in this respect. However, it does encourage the author to spend longer in the beginning at the straight slide-rule work. As it happens, I get most of the fun out of working out the physical and chemical nature of a planet or solar system, and then dreaming up life forms which might reasonably evolve under such conditions. The story (obviously, as some critics have been known to remark) comes afterward. My excuse for using this general technique, if one is needed, is twofold. First, I find it more fun.

This will carry smaller weight for the author who is writing for a living. Second, it is not unusual for the nature of the planet and its life forms, once worked out, to suggest story events or even an entire plot line which would never otherwise have occurred to me. This fact should carry some weight even with the more fantasy-oriented writer, who cares less about “realism.” I do have to admit that realism, or at least consistency, is a prime consideration with me; and as I implied some pages back with the Abraham Lincoln metaphor, even the most fantastic story can jar the most tolerant reader if the inconsistency is crude enough—anachronism is only one form of inconsistency. This sort of realism in life design has to be on at least two levels: biochemical and mechanical. BIOCHEMICAL REALISM It is true that we do not yet know all the details of how even the simplest life forms work. It is still defensible to build for story purposes a creature that drinks hydrazine, and say that no one can prove this impossible.

Beyond a certain point, however, I have to dismiss this as ducking out the easy way—sometimes justifiable for storytelling purposes, but jarring on the scientific sensibility. Some facts of life are very well known indeed, and to contradict them, a very good excuse and very convincing logic are needed. For example, any life form converts energy from one form to another. On our own planet, the strongest and most active creatures use the oxygen in the atmosphere to convert food materials to carbon dioxide and water. The chemical reactions supply the needed energy. Obviously, the available oxygen would be quickly used up if there were not some other set of reactions to break down the water and carbon dioxide (actually it's the water, on this planet) to replace what is exhausted. It takes as much energy (actually more must be supplied, since no reaction is completely efiicient) to break up a molecule into its elements as is released by forming it from these elements, and any ecological system must have a long-term energy base.

On this planet, as is common knowledge, the base is sunlight. There seems no need here to go into the very complicated details; few people get through high school these days (I’d like to believe) without at least a general idea of photosynthesis. In passing, some people have the idea that fish violate this basic rule, and are some sort of perpetual motion machine, because they “breathe water.” Not so; fish use the elemental O 2 gas supplied as usual by photosynthesis and dissolved in water, not the O in the H 2O. Aquarium suppliers are perfectly justified in selling air pumps; they are not exploiting the innocent fish-fanciers. Substitutes for free oxygen in energy-releasing reactions are perfectly possible chemically, and as far as anyone can tell should be possible biologically (indeed, some Earthly life forms do use other reactions). There is no chemical need for these substitutes even to be gases; but if the story calls for a nonhuman character to be drowned or strangled, obvious gaseous candidates are fluorine and chlorine.

The former can run much more energetic reactions than even oxygen, while chlorine compares favorably with the gas we are all hooked on. (That last seems a justified assumption about the present readers. If it is wrong, please come and introduce yourself!) Neither chlorine nor fluorine occurs free on this planet; but, as pointed out already, neither would oxygen if earthly life were not constantly replenishing it by photosynthesis.

It has been pointed out that both these gases are odd-numbered elements and therefore in shorter universal supply than oxygen. This may well be true; but if some mad scientist were to develop a microorganism able to photosynthesize free chlorine from the chloride ion in Earth’s ocean, it wouldn’t have to do a very complete job to release as much of this gas as we now have of oxygen. Breaking down ten percent or so of the ocean salt would do the trick.

Present-day biological engineering is probably not quite up to this job yet, but if you want to use the idea in a story be my guest. I don’t plan to use it myself; the crazy-scientist story is old hat now except in frankly political literature, and even the germ-from-space has been pretty well worked to death in the last forty years. As mentioned, there is no chemical reason why the energy-producing reactants have to include gases at all. Oxidizing a pound of sugar with nitric acid will yield more energy than oxidizing the same pound with oxygen (if this seems improbable at first glance, remember the bond energy of the N 2 molecule which is one of the products of the first reaction). True, raw concentrated nitric acid is rather hard on most if not all Terrestrial tissues; but we do handle hydrochloric acid—admittedly in rather dilute form in spite of the antacid-tablet ads—in our own digestive systems.

I see little difiiculty in dreaming up a being able to store and utilize strong oxidizers in its system. The protective mucus our own stomachs use is only one of the possibilities. Many chemical sources of energy are therefore possible in principle for our life forms; but one should be reasonably aware of the chemistry involved.

Water or iron oxide would not be good fuels under any reasonable circumstances; there are admittedly some energy-yielding reactions involving these, but they call for special and unlikely reactants like sodium or fluorine—and if those reactants are around, we could get much more energy by using them on other substances. To get more fundamental, sunlight is not the only conceivable energy base for an ecological pyramid. It is, however, by far the most likely, assuming the planet in question has a sun. Remember, the energy source must not only be quantitatively large enough; it must be widely available in both space and time, so that life can originate and evolve to complex forms. Radioactivity and raw volcanic heat are both imaginable, but the first demands rather unusual conditions if much of it is to be on hand. Vulcanism, if Earth is a fair example, tends to be restricted in space at any one time and in time at any one location, a discouraging combination.

Also, radioactive energy in its most direct form comes in high-energy quanta, furnishing an additional complication to the molecular architecture problem to be considered next. It seems pretty certain that life, as well as needing energy, must be of complex structure. It has to do too many things for a simple machine.

An organism must be able to absorb the chemicals needed for its energy, and carry out at the desired rate the reactions which they undergo. It must develop and repair its own structure (immortal, invulnerable, specially created beings are conceivable, but definitely outside the realm of this discussion). It must reproduce its own structure, and therefore keep on file a complete set of specifications—which must itself be reproducible. Whatever mystical, symbolic, and figurate resemblances there may be between a candle flame and a living creature, the concrete differences between them seem to me to constitute a non-negotiable demand for extreme complexity in the latter. On Earth, this complexity involves the phosphate-sugar-base polymers called popularly DNA and RNA for specifications, polypeptide and polysaccharide structures for most of the machinery, and—perhaps most fundamentally—the hydrogen bond to provide structural links which can be changed around as needed without the need for temperatures high enough to ruin the main framework. I see no reason why other carbon compounds could not do the jobs of most of these, though I cannot offhand draw formulas for the alternates. The jobs in general depend on the shapes of the molecules, or perhaps more honestly the shapes of the force fields around them; these could presumably be duplicated closely enough by other substances.

I am rather doubtful that the cruder substitutions suggested by various writers, such as that of silicon for carbon, would actually work, though of course I cannot be sure that they wouldn’t. We have the fact that on Earth, with silicon many times more plentiful than carbon, life uses the latter. The explanations which can be advanced for this fact seem to me to be explanations as well of why silicon won’t work in life forms. (To be more specific: silicon atoms are large enough to four-coordinate with oxygen, and hence wind up in hard, crystalline, insoluble macromolecular structures—the usual run of silicate minerals. The smaller carbon atom, able to react with not more than three oxygens at once, was left free to form the water-reactive carbon dioxide gas.) True, some Earthly life such as scouring rushes, basket sponges, and foraminifera use silicon compounds in skeletal parts; but not, except in trace amounts, in active life machinery. I also doubt that any other element could do the job of hydrogen, which I am inclined to regard as “the” essential life element, rather than the more popular carbon.

Life machinery is complex, but it must have what might be called “moving parts” —structures which have to be altered in shape, or connected now one way and now another. A chemical bond weak enough to be changed without affecting the rest of the machine seems a necessity—a gasoline engine would be hard to design if springs didn’t exist and a cutting torch were needed to open the valves each cycle. The hydrogen bond (I don’t propose to explain what this is; if you don’t know, consult any beginning chemistry text) is the only thing I know of which meets this need on the molecular level. This, however, is not much of a science fiction problem. Something like 999 out of every 1000 atoms in the universe are hydrogen atoms; even Earth, which seems to be one of the most thoroughly dehydrogenated objects in the observable part of space, has all it needs for an extensive collection of life forms.

I suspect it will generally be easier for an author to use hydrogen in his homemade life forms than to work out a credible substitute. To finish with the fundamental-structure level, one must admit that very complex electric and magnetic field structures other than those supplied ready-formed by atoms and molecules are conceivable. At this point, it really is necessary to fall back on the “we can’t say it’s impossible” excuse. Personally I would develop such life forms only if my story demanded of them some ability incompatible with ordinary matter, such as traveling through a telephone wire or existing without protection both in the solar photosphere and a cave on Pluto. At this point, simple scientific realism fades away, and I must bow out as an expert. It’s not that I’m above doing it; it’s just that practically anyone else could do it equally well. MECHANICAL REALISM The other principal basis for believability of life forms lies in the field of simple mechanics, much more common sense than biochemistry.

For example, in spite of Edgar Rice Burroughs’s, a fast-running creature is far more likely to have a few long legs than a lot of short ones. Whether muscle tissue on Planet X is stronger or weaker than on Earth, muscular effort will be more efiiciently applied by fewer, longer strokes. Even if the evolutionary background for some reason started off with the ten legs (e.g., high gravity), I would expect an organism specializing in speed to develop two, or perhaps four, of them to greater length and either have the others degenerate or put them to other uses as the generations rolled on. On the same general principle, if the creature lives on grass or the local ecological equivalent, it will probably not have much of a brain. If it doesn’t have to catch food or climb trees, it will lack any equivalent of a hand—in short, any anatomical part an organism has should either be useful to that creature in its current life, or be the degenerate remnant of something useful to its remote ancestors. Exceptions to this rule among Earthly life forms are hard to find, and may be only apparent; we simply don’t know the purpose of the organ in question. A former example was the “sail” on the backs of some Permian reptiles, now believed to be a temperature control device.

In addition to being useful itself, a structure must have been at least slightly useful through its early stages of development; it is hard to believe that a single mutation would produce a completely developed ear, but any ability to sense pressure variations would clearly be useful to an animal. Creatures must have existed showing development all the way from a slightly refined sense of touch to the present organ capable of detecting and recognizing a tiger’s footfall in a windy forest—or an out-of-tune flute in an orchestra. Similarly with the eye. There are now alive on Earth creatures with light-sensitive organs ranging from the simple red spot of the single-celled Euglena, through pinhole cameras with complex retinas (some cephalopods), to the lens-and-iris-equipped diffraction-limited organ of most mammals and birds, complete with automatic focusing. There are also examples of parallel evolution which were good enough to help their owners survive all the way along the route: the compound mosaic-lens eyes of arthropods and, I have heard, at least one organism that scans the image of a single lens by moving a single retinal nerve over the field. But eyes and ears are hardly original enough for a really imaginative science fiction story. What other long-range senses might an organism evolve?

Could an intelligent species develop without any such sense? If so, what would be that creature’s conception of the universe? How, if at all, could sighted and hearing human beings communicate with it? The first question at least can be partially answered without recourse to mysticism.

Magnetic fields do exist, as do electric ones. Certainly some creatures can sense the latter directly (you can yourself, for that matter; bring your hand close to a highly charged object and feel what happens to the fine hairs on your skin). There is some evidence that certain species of birds can detect the earth’s magnetic field. Sound is already used in accordance with its limitations, as is scent. A gravity-sense other than the one we now use for orientation would probably not be discriminating enough, though I could certainly be wrong (read up on lunar mascons if you don’t see what I mean by lack of discrimination). It is a little hard to envision what could be detected by a magnetic sense, and how its possessor would imagine the universe. Most substances on this planet have practically no effect on a magnetic field, and this is what makes me a little doubtful about the birds mentioned above.

I can see the use of such a sense in navigation for a migratory species, but I have trouble thinking through its evolutionary development. Perhaps on a planet with widely distributed ferromagnetic material, the location of which is of life-and-death importance to the life forms, it would happen; maybe our Regulus IV character who can dissolve iron needs it for biochemical reasons. The important point, from which we may have been wandering a trifle, is not whether I can envision such a situation in detail, but whether the author of the story can do so, and thereby avoid having to invent ad hoc a goose which lays golden eggs. If the life form in question has hearing but no sight, all right; but it should not be able to thread a needle with the aid of sonic perception.

Sound waves short enough to have that kind of resolving power would demand a good deal of energy to produce, would have very poor range in air, and would incidentally be decidedly dangerous to human explorers. Of course, a story could be built on the unfortunate consequences of the rnen who were mowed down by what they thought must be a death ray, when the welcoming committee was merely trying to take a good look Sound does have the advantage of being able to diffract around obstacles, so that straight-line connection is not needed; light (that is, light visible to human beings) is of such short wavelength that diffraction efiects are minor. This means that the precise direction of origin of a sound ray cannot be well determined, while a good eye can measure light’s direction to a small fraction of a degree.

On Earth, we both eat and keep this particular piece of cake, since we have evolved both sight and hearing. Scent seems to have all the disadvantages and none of the advantages, as a long-range sense. However, under special circumstances even a modified nose may fill the need. In a story of my own some years ago (“,” Astounding Science Fiction, September 1945), I assumed an airless planet, so that molecules could ditfuse in nearly straight lines. The local sense organs were basically pinhole cameras, with the retinal mosaic formed of olfactory cells. Since the beings in question were not intelligent, the question of what sort of universe they believed in did not arise. Granting the intelligence, it would have been—would still be, indeed—interesting to work out their cosmology.

Naturally, the first few hours are spent wondering whether and how they could fill the intellectual gaps imposed by their lack of sight and hearing. Then, of course, the intelligent speculator starts wondering what essential details are missing from our concept of the universe, because of our lack of the sense of (you name it). This, for what my opinion is worth, is one of the best philosophical excuses for the practice of science fiction—if an excuse is needed. The molecule-seers presumably lack all astronomical data; what are we missing? This question, I hope I needn’t add, is not an excuse to go off on a mystical kick, though it is one which the mystics are quite reasonably fond of asking (and then answering with their own version of Truth). The human species has, as a matter of fact, done a rather impressive job of overcoming its sensory limitations, though I see no way of ever being sure when the job is done.

Philosophy aside, there are many more details of shape to be considered for nonhuman beings. Many of the pertinent factors have been pointed out by other writers, such as L. Sprague deCamp (“ for,” Astounding Science Fiction, May-June, 1939). DeCamp reached the conclusion that an intelligent life form would have to wind up not grossly different in structure from a human being—carrying its sense organs high and close to the brain, having a limited number of limbs with a minimum number of these specialized for locomotion and the others for manipulation, having a rigid skeleton, and being somewhere between an Irish terrier and a grizzly bear in size.

The lower size limits was set by the number of cells needed for a good brain, and the upper one by the bulk of body which could be handled by a brain without overspecialization. Sprague admitted both his estimates to be guesses, but I have seen no more convincing ones since. Whenever I have departed greatly from his strictures in my own stories, I have always felt the moral need to supply an excuse, at least to myself. The need for an internal skeleton stems largely from the nature of muscle tissue, which can exert force only by contracting and is therefore much more effective with a good lever system to work with. I belittle neither the intelligence nor the strength of the octopus; but and most other writers of undersea adventure, the creature’s are not all that effective as handling organs. I don’t mean that the octopus and his kin are helpless hunks of meat; but if I had my choice of animals I was required to duel to the death, I would pick one of this tribe rather than one of their bonier rivals, the barracuda or the moray eel, even though neither of the latter have any prehensile organs but their jaws. (If any experienced scuba divers wish to dispute this matter of taste, go right ahead.

I admit that so far, thank goodness, I am working from theory on this specific matter.) This leads to a point which should be raised in any science fiction essay. I have made a number of quite definite statements in the preceding pages, and will make several more before finishing this chapter. Anyone with the slightest trace of intelligent critical power can find a way around most of these dicta by setting up appropriate situations. I wouldn’t dream of objecting; most of my own stories have developed from attempts to work out situations in which someone who has laid down the law within my hearing would be wrong. The Hunter in was a deliberate attempt to get around Sprague’s minimum-size rule.

By variable gravity. If no one has the urge, imagination, and knowledge to kick specific holes in the things I say here, my favorite form of relaxation is in danger of going out with a whimper. If someone takes exception to the statement that muscles can only pull, by all means do something about it. We know a good deal about Earthly muscle chemistry these days; maybe a pushing cell could be worked out.

I suspect it would need a very strong cell wall, but why not? Have fun with the idea. If you can make it plausible, you will have destroyed at a stroke many of the currently plausible engineering limitations to the shapes and power of animals.

I could list examples for the rest of my available pages, but you should have more fun doing it yourself. There is a natural temptation to make one’s artificial organisms as weird as possible in looks and behavior. Most authors seem to have learned that it is extremely hard to invent anything stranger than some of the life forms already on our planet, and many writers as a result have taken to using either these creatures as they are, or modifying them in size and habit, or mixing them together. The last, in particular, is not a new trick; the and have been with us for some time. With our present knowledge, though, we have to be careful about the changes and mixtures we make., for example, will have to remain mythological. Even if we could persuade a horse to grow wings (feathered or not), Earthly muscle tissue simply won’t fly a horse (assuming, of course, that the muscle is going along for the ride).

Also, the horse would have to extract a great deal more energy than it does from its hay diet to power the flight muscles even if it could find room for them in an equine anatomy. Actually, the realization that body engineering and life-style are closely connected is far from new. There is a story about, a naturalist of the late eighteenth and early nineteenth centuries. It seems that one night his students decided to play a practical joke, and one of them dressed up in a conglomeration of animal skins, including that of a deer. The disguised youth then crept into the baron’s bedroom and aroused him by growling, “Cuvier, wake up!

I am going to eat you!” The baron is supposed to have opened his eyes, looked over his visitor briefly, closed his eyes again and rolled over muttering, “Impossible! You have horns and hooves.” A large body of information, it would seem, tends to produce opinions in its possessor’s mind, if not always correct ones. The trick of magnifying a normal creature to menacing size is all too common. The giant amoeba is a familar example; monster insects (or whole populations of them) even more so. It might pay an author with this particular urge to ask himself why we don’t actually have such creatures around. There is likely to be a good reason, and if he doesn’t know it perhaps he should do some research. In the case of both amoeba and insect, the so-called “” law is the trouble.

Things like strength of muscle and rate of chemical and heat exchange with the environment depend on surface or cross-section area, and change with the square of linear size; Swift's Brobdingnagians would therefore have a hundred times the strength and oxygen intake rate of poor Gulliver. Unfortunately the mass of tissue to be supported and fed goes up with the cube of linear dimension, so the giants would have had a thousand times Gulliver’s weight. It seems unlikely that they could have stood, much less walked (can you support ten times your present weight?). This is why a whale, though an air breather, suffocates if he runs ashore; he lacks the muscular strength to expand his chest cavity against its own weight. An ant magnified to six-foot length would be in even worse trouble, since she doesn’t have a mammal’s supercharger system in the first place, but merely a set of air pipes running through her system. Even if the mad scientist provided his giant ants with oxygen masks, I wouldn’t be afraid of them. It is only because they are so small, and their weight has decreased even faster than their strength, that insects can perform the “miraculous” feats of carrying dozens of times their own weight or jumping hundreds of times their own length.

Ariane Moffatt Ma Ziplining. This would have favored Swift’s Lilliputians, who would have been able to make some remarkable athletic records if judged on a strictly linear scale. That is, unless they had to spend too much time in eating to offset their excessive losses of body heat Really small creatures, strong as they may seem, either have structures that don’t seem to mind change in temperature too much (insects, small reptiles), or are extremely well insulated (small birds), or have to eat something like their own weight in food each day (shrew, hummingbird). There seems reason to believe that at least with Earthly biochemistry, the first and last of these weaknesses do not favor intelligence.

A rather similar factor operates against the idea of having a manlike creature get all his energy from sunlight, plant style. This was covered years ago by V. Eulach (“,” Astounding Science Fiction, October 1956), who pointed out that a man who tries to live like a tree is going to wind up looking much like one. He will have to increase his sunlight-intercepting area without greatly increasing his mass (in other words, grow leaves), cut down his energy demands to what leaves can supply from sunlight’s one-and-a-half-horse-power-per-square-yard (become sessile), and provide himself with mineral nutrients directly from the soil, since he can’t catch food any more (grow roots!). Of course, we can get around some of this by hypothesizing a hotter, closer sun, with all the attendant complications of higher planet temperature.

This is fun to work out, and some of us do it, but remember that a really basic change of this sort affects everything in the ecological pyramid sitting on that particular energy base—in other words, all the life on the planet. It may look from all this as though a really careful and conscientious science fiction writer has to be a junior edition of the Almighty. Things are not really this bad. I mentioned one way out a few pages ago in admitting there is a limit to the detail really needed. The limit is set not wholly by time, but by the fact that too much detail results in a Ph.D.

Thesis—perhaps a fascinating one to some people, but still a thesis rather than a story. I must admit that some of us do have this failing, which has to be sharply controlled by editors.

Perhaps the most nearly happy-medium advice that can be given is this: Work out your world and its creatures as long as it remains fun; then Write your story, making use of any of the details you have worked out which help the story. Write off the rest of the development work as something which built your own background picture—the stage setting, if you like—whose presence in your mind will tend to save you from the more jarring inconsistencies (I use this word, very carefully, rather than errors). Remember, though, that among your readers there will be some who enjoy carrying your work farther than you did. They will find inconsistencies which you missed; depend on it. Part of human nature is the urge to let the world know how right you were, so you can expect to hear from these people either directly or through fanzine pages.

Don’t let it Worry you. Even if he is right and you are wrong, he has demonstrated unequivocally that you succeeded as a storyteller.

You gave your audience a good time. Wikipedia has a nice article on In a science essay ', Isaac Asimov notes that life on Terra is based on proteins dissolved in water solvent. He points out some other possibilities. Note that the 'temperature' column has the information needed to set the borders of a solar system's for that particular biochemistry. Temperatures assume the planet has about 1 atmosphere worth of pressure.

In Solvent Temperature at 1 Atm Notes Fluorosilicones in Fluorosilicones 400°? C Silanes (chains of silicon atoms) are too unstable. Silicones (chains alternating silicon and oxygen atoms) are more suitable for making 'silicon life' protein analogues.

Notes that such life will consume carbon dioxide (and other carbon compounds) out of the air, combining it with silicon to create complex silicone compounds. Oxygen will be released but that will immediately combine with silicon to make silicon dioxide sand.

The atmosphere will become depeleted in carbon dioxide. This might cool the planet off enough that fluorocarbon-sulfur life will take over the planet. Fluorocarbons in Molten Sulfur 113° to 445° C Earth proteins are too unstable at liquid sulfur temperatures. They can be stabilized by substituting fluorine atoms for hydrogen atoms, resulting in complex fluorocarbons.

Notes that such life forms will probably evolve in an atmosphere poor in oxygen but rich in fluorine. However, such life will create atmospheres with oxygen as they release oxygen from carbon dioxide+sulfur dioxide as their metabolism creates complex fluorocarbon molecules. There actually might be enough oxygen in the atmosphere for humans to breath (but the temperature would kill them). In Water 0° to 100° C Because water is hydrogenated oxygen, the proteins will have to have more oxygen than nitrogen in their make up. This is 'life as we know it.' Pretty much all life on Terra falls under this catagory.

Notes that such life will consume carbon dioxide out of the atmosphere and release oxygen, thus converting the planet's primordial atmosphere into a biologic oxygen containing atmosphere. Proteins in Liquid -77.7° C to -33.4° C Because ammonia is hydrogenated nitrogen, the proteins will have more nitrogen than oxygen in their make up. Earth proteins are too stable at liquid ammonia temperatures, ammonia life proteins will have to be more unstable than their Earth analogues.

Notes that such life forms will probably require a planet with a methane-ammonia atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water. The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and become much lower in pressure. Lipids in Liquid Methane -183.6° C to -161.6° C Polar liquids will not dissolve non-polar substances and vice versa (oil and water don't mix).

Proteins are polar, so they won't dissolve in liquid methane. Complex protein-like polylipids will have to be used instead. Notes that such life forms will probably require a planet with a methane-hydrogen atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water while the oxygen will react with hydrogen to produce more water.

The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and hydrogen thus becoming much lower in pressure. Lipids in Liquid Hydrogen -253° C to -240° C Liquid hydrogen is also non-polar, so polylipids will be needed. Notes that the temperature will be much higher in the immense pressures of a gas giant world. (1968) In classic science fiction, the buzz-word was '. Life on Terra is based on Carbon, since carbon can join with not one, not two, not even three, but a whopping four other atoms.

This allows the construction of complex molecules like proteins and DNA, a requirement for living creatures. The only other element that can do this is Silicon, so the SF writers seized it. They are also fond of harping on the fact that while most carbon-based animals on Terra exhale gaseous carbon dioxide, a poor silicon-based critter would breath out silicon dioxide, i.e.,sand. In 'A Martian Odyssey' by Stanley Weinbaum is a silicon life creature that 'exhales' bricks of silicon dioxide, which it uses to build a pyramid around itself. Other chemical elements that are not impossible as the basis for alien life forms include,,, and. There are even more.

There are several possibilities for the. An example of electronic life is the superconducting mentality in Sir Arthur C. One of the odder aliens is the Qax from Stephen Baxter's Timelike Infinity. Their 'bodies' are organized clusters of millions of tiny whirlpools in still ponds.

Another odd one was the. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.

So we must strike beyond physiology and reach into chemistry, saying that all life is made up of a directing set of nucleic acid molecules which controls chemical reactions through the agency of proteins working in a watery medium. There is more, almost infinitely more, to the details of life, but I am trying to strip it to a basic minimum. For life-as-we-know-it, water is the indispensable background against which the drama is played out, and nucleic acids and proteins are the featured players. Hence any scientist, in evaluating the life possibilities on any particular world, instantly dismisses said world if it lacks water; or if it possesses water outside the liquid range, in the form of ice only or of steam only. (You might wonder, by the way, why I don't include oxygen as a basic essential.

I don't because it isn't. To be sure, it is the substance most characteristically involved in the mechanics by which most life forms evolve energy, but it is not invariably involved. There are tissues in our body that can live temporarily in the absence of molecular oxygen, and there are microorganisms that can live indefinitely in the absence of oxygen. Life on earth almost certainly developed in an oxygen-free atmosphere, and even today there are microorganisms that can live only in the absence of oxygen.

No known life form on earth, however, can live in the complete absence of water, or fails to contain both protein and nucleic acid.) In order to discuss life-not-as-we-know-it, let's change either the background or the feature players. Background first! Water is an amazing substance with a whole set of unusual properties which are ideal for life-as-we-know-it. So well fitted for life is it, in fact, that some people have seen in the nature of water a sure sign of Divine providence. This, however, is a false argument, since life has evolved to fit the watery medium in which it developed. Life fits water, rather than the reverse. Can we imagine life evolving to fit some other liquid, then, one perhaps not too different from water?

The obvious candidate is ammonia. Ammonia is very like water in almost all ways. Whereas the water molecule is made up of an oxygen atom and two hydrogen atoms (H 2O) for an atomic weight of 18, the ammonia molecule is made up of a nitrogen atom and three hydrogen atoms (NH 3) for an atomic weight of 17. Liquid ammonia has almost as high a heat of evaporation, almost as high a versatility as a solvent, almost as high a tendency to liberate a hydrogen ion. In fact, chemists have studied reactions proceeding in liquid ammonia and have found them to be quite analogous to those proceeding in water, so that an 'Ammonia chemistry' has been worked out in considerable detail. Ammonia as a background to life is therefore quite conceivable — but not on earth. The temperatures on earth are such that ammonia exists as a gas.

Its boiling point at atmospheric pressure is -33.4° C. (-28° F.) and its freezing point is -77.7° C. But other planets? In 1931, the spectroscope revealed that the atmosphere of Jupiter, and, to a lesser extent, of Saturn, was loaded with ammonia. The notion arose at once of Jupiter being covered by huge ammonia oceans. To be sure, Jupiter may have a temperature not higher than -100° C. (-148° F.), so that you might suppose the mass of ammonia upon it to exist as a solid, with atmospheric vapor in equilibrium.

If Jupiter were closer to the sun. The boiling point I have given for ammonia is at atmospheric pressure — earth's atmosphere. At higher pressures, the boiling point would rise, and if Jupiter's atmosphere is dense enough and deep enough, ammonia oceans might be possible after all. An objection that might, however, be raised against the whole concept of an ammonia background for life, rests on the fact that living organisms are made up of unstable compounds that react quickly, subtly and variously.

The proteins that are so characteristic of life-as-we-know-it must consequently be on the edge of instability. A slight rise in temperature and they break down. A drop in temperature, on the other hand, might make protein molecules too stable. At temperatures near the freezing point of water, many forms of non-warm-blooded life become sluggish indeed.

In an ammonia environment with temperatures that are a hundred or so Centigrade degrees lower than the freezing point of water, would not chemical reactions become too slow to support life? The answer is twofold. In the first place, why is 'slow' to be considered 'too slow?' Why might there not be forms of life that live at slow motion compared to ourselves? A second and less trivial answer is that the protein structure of developing life adapted itself to the temperature by which it was surrounded. Had it adapted itself over the space of a billion years to liquid ammonia temperatures, protein structures might have been evolved that would be far too unstable to exist for more than a few minutes at liquid water temperatures, but are just stable enough to exist conveniently at liquid ammonia temperatures. These new forms would be just stable enough and unstable enough at low temperatures to support fast-moving forms of life.

Nor need we be concerned over the fact that we can't imagine what those structures might be. Suppose we were creatures who lived constantly at a temperature of a dull red heat (naturally with a chemistry fundamentally different from that we now have). Could we under those circumstances know anything about earth-type proteins? Could we refrigerate vessels to a mere 25° C., form proteins and study them?

Would we ever dream of doing so, unless we first discovered life forms utilizing them? Anything else besides ammonia now? Well, the truly common elements of the universe are hydrogen, helium, carbon, nitrogen, oxygen and neon.

We eliminate helium and neon because they are completely inert and take part in no reactions. In the presence of a vast preponderance of hydrogen throughout the universe, carbon, nitrogen and oxygen would exist as hydrogenated compounds. In the case of oxygen, that would be water (H 2O), and in the case of nitrogen, that would be ammonia (NH 3). Both of these have been considered. That leaves carbon, which, when hydrogenated, forms methane (CH 4).There is methane in the atmosphere of Jupiter and Saturn, along with ammonia; and, in the still more distant planets of Uranus and Neptune, methane is predominant, as ammonia is frozen out. This is because methane is liquid over a temperature range still lower than that of ammonia. It boils at -161.6° C.

(-259° F.) and freezes at -182.6° C. (-297° F.) at atmospheric pressure. Could we then consider methane as a possible background to life with the feature players being still more unstable forms of protein? Unfortunately, it's not that simple. Ammonia and water are both polar compounds; that is, the electric charges in their molecules are unsymmetrically distributed.

The electric charges in the methane molecule are symmetrically distributed, on the other hand, so it is a non-polar compound. Now, it so happens that a polar liquid will tend to dissolve polar substances but not nonpolar substances, while a nonpolar liquid will tend to dissolve nonpolar substances but not polar ones.

Thus water, which is polar, will dissolve salt and sugar, which are also polar, but will not dissolve fats or oils (lumped together as 'lipids' by chemists), which are nonpolar. Hence the proverbial expression, 'Oil and water do not mix.' On the other hand, methane, a nonpolar compound, will dissolve lipids but will not dissolve salt or sugar.

Proteins and nucleic acids are polar compounds and will not dissolve in methane. In fact, it is difficult to conceive of any structure that would jibe with our notions of what a protein or nucleic acid ought to be that would dissolve in methane. If we are to consider methane, then, as a background for life, we must change the feature players. To do so, let's take a look at protein and nucleic acid and ask ourselves what it is about them that makes them essential for life. Well, for one thing, they are giant molecules, capable of almost infinite variety in structure and therefore potentially possessed of the versatility required as the basis of an almost infinitely varying life. Is there no other form of molecule that can be as large and complex as proteins and nucleic acids and that can be nonpolar, hence soluble in methane, as well?

The most common nonpolar compounds associated with life are the lipids, so we might ask if it is possible for there to exist lipids of giant molecular size. Such giant lipid molecules are not only possible; they actually exist.

Brain tissue, in particular, contains giant lipid molecules of complex structure (and of unknown function). There are large 'lipoproteins' and 'proteolipids' here and there which are made up of both lipid portions and protein portions combined in a single large molecule. Man is but scratching the surface of lipid chemistry; the potentialities of the nonpolar molecule are greater than we have, until recent decades, realized. Remember, too, that the biochemical evolution of earth's life has centered about the polar medium of water.

Had life developed in a nonpolar medium, such as that of methane, the same evolutionary forces might have endlessly proliferated lipid molecules into complex and delicately unstable forms that might then perform the functions we ordinarily associate with proteins and nucleic acids. Working still further down on the temperature scale, we encounter the only common substances with a liquid range at temperatures below that of liquid methane.

These are hydrogen, helium, and neon. Again, eliminating helium and neon, we are left with hydrogen, the most common substance of all. (Some astronomers think that Jupiter may be four-fifths hydrogen, with the rest mostly helium — in which case good-by ammonia oceans after all.) Hydrogen is liquid between temperatures of -253° C. (-423° F.) and -259° C.

(-434° F.), and no amount of pressure will raise its boiling point higher than -240° C. This range is only twenty to thirty Centigrade degrees over absolute zero, so that hydrogen forms a conceivable background for the coldest level of life. Hydrogen is nonpolar, and again it would be some sort of lipid that would represent the featured player.

So far the entire discussion has turned on planets colder than the earth. What about planets warmer? To begin with, we must recognize that there is a sharp chemical division among planets. Three types exist in the solar system and presumably in the universe as a whole. On cold planets, molecular movements are slow, and even hydrogen and helium (the lightest and therefore the nimblest of all substances) are slow-moving enough to be retained by a planet in the process of formation. Since hydrogen and helium together make up almost all of matter; this means that a large planet would be formed. Jupiter, Saturn, Uranus and Neptune are the examples familiar to us.

On warmer planets, hydrogen and helium move quickly enough to escape. The more complex atoms, mere impurities in the overriding ocean of hydrogen and helium, are sufficient to form only small planets. The chief hydrogenated compound left behind is water, which is the highest-boiling compound of the methane-ammonia-water trio and which, besides, is most apt to form tight complexes with the silicates making up the solid crust of the planet. Worlds like Mars, earth, and Venus result.

Here, ammonia and methane forms of life are impossible. Firstly, the temperatures are high enough to keep those compounds gaseous.

Secondly, even if such planets went through a super-ice-age, long aeons after formation, in which temperatures dropped low enough to liquefy ammonia or methane, that would not help. There would be no ammonia or methane in quantities sufficient to support a world-girdling life form. Imagine, next a world still warmer than our medium trio: a world hot enough to lose even water. The familiar example is Mercury. It is a solid body of rock with little, if anything, in the way of hydrogen or hydrogen-containing compounds. Does this eliminate any conceivable form of life that we can pin down to existing chemical mechanisms? Not necessarily.

There are nonhydrogenous liquids, with ranges of temperature higher than that of water. The most common of these, on a cosmic scale, has a liquid range from 113° C.

(235° F.) to 445° C. (833° F.); this would fit nicely into the temperature of Mercury's sunside. But what kind of featured players could be expected against such a background? So far all the complex molecular structures we have considered have been ordinary organic molecules; giant molecules, that is, made up chiefly of carbon and hydrogen, with oxygen and nitrogen as major 'impurities' and sulfur and phosphorus as minor ones.

The carbon and hydrogen alone would make up a nonpolar molecule; the oxygen and nitrogen add the polar qualities. In a watery background (oxygen-hydrogen) one would expect the oxygen atoms of tissue components to outnumber the nitrogen atoms, and on earth this is actually so. Against an ammonia background, I imagine nitrogen atoms would heavily outnumber oxygen atoms. The two subspecies of proteins and nucleic acids that result might be differentiated by an O or an N in parentheses, indicating which species of atom was the more numerous. The lipids, featured against the methane and hydrogen backgrounds, are poor in both oxygen and nitrogen and are almost entirely carbon and hydrogen, which is why they are nonpolar.

But in a hot world like Mercury, none of these types of compounds could exist. No organic compound of the types most familiar to us, except for the very simplest, could long survive liquid sulfur temperatures. In fact, earthly proteins could not survive a temperature of 60° C. For more than a few minutes. How then to stabilize organic compounds? The first thought might be to substitute some other element for hydrogen, since hydrogen would, in any case, be in extremely short supply on hot worlds. So let's consider hydrogen.

The hydrogen atom is the smallest of all atoms and it can be squeezed into a molecular structure in places where other atoms will not fit. Any carbon chain, however intricate, can be plastered round and about with small hydrogen atoms to form 'hydrocarbons.' Any other atom, but one, would be too large. And which is the 'but one?' Well, an atom with chemical properties resembling those of hydrogen (at least as far as the capacity for taking part in particular molecular combinations is concerned) and one which is almost as small as the hydrogen atom, is that of fluorine. Unfortunately, fluorine is so active that chemists have always found it hard to deal with and have naturally turned to the investigation of tamer atomic species. This changed during World War II.

It was then necessary to work with uranium hexafluoride, for that was the only method of getting uranium into a compound that could be made gaseous without trouble. Uranium research had to continue (you know why), so fluorine had to be worked with, willy-nilly.

As a result, a whole group of 'fluorocarbons,' complex molecules made up of carbon and fluorine rather than carbon and hydrogen, were developed, and the basis laid for a kind of fluoro-organic chemistry. To be sure, fluorocarbons are far more inert than the corresponding hydrocarbons (in fact, their peculiar value to industry lies in their inertness) and they do not seem to be in the least adaptable to the flexibility and versatility required by life forms. However, the fluorocarbons so far developed are analogous to polyethylene or polystyrene among the hydro-organics. If we were to judge the potentialities of hydro-organics only from polyethylene, I doubt that we would easily conceive of proteins. No one has yet, as far as I know, dealt with the problem of fluoroproteins or has even thought of dealing with it — but why not consider it? We can be quite certain that they would not be as active as ordinary proteins at ordinary temperatures. But on a Mercury-type planet, they would be at higher temperatures, and where hydro-organics would be destroyed altogether, fluoro-organcs might well become just active enough to support life, particularly the fluoro-organics that life forms are likely to develop.

Such fluoro-organic-in-sulfur life depends, of course, on the assumption that on hot planets, fuorine, carbon and sulfur would be present in enough quantities to make reasonably probable the development of life forms by random reaction over the life of a solar system. Each of these elements is moderately common in the universe, so the assumption is not an altogether bad one. But, just to be on the safe side, let's consider possible alternatives. Suppose we abandon carbon as the major component of the giant molecules of life. Are there any other elements which have the almost unique property of carbon — that of being able to form long atomic chains and rings — so that giant molecules reflecting life's versatility can exist? The atoms that come nearest to carbon in this respect are boron and silicon, boron lying just to the left of carbon on the periodic table (as usually presented) and silicon just beneath it. Of the two, however, boron is a rather rare element.

Its participation in random reactions to produce life would be at so slow a rate, because of its low concentration in the planetary crust, that a boron-based life formed within a mere five billion years is of vanishingly small probability. That leaves us with silicon, and there, at least, we are on firm ground. Mercury, or any hot planet, may be short on carbon, hydrogen and fluorine, but it must be loaded with silicon and oxygen, for these are the major components of rocks. A hot planet which begins by lacking silicon and oxygen as well, just couldn't exist because there would be nothing left in enough quantity to make up more than a scattering of nickel-iron meteorites. Silicon can form compounds analogous to the carbon chains. Hydrogen atoms tied to a silicon chain, rather than to a carbon chain, form the 'silanes.' Unfortunately, the silanes are less stable than the corresponding hydrocarbons and are even less likely to exist at high temperatures in the complex arrangements required of molecules making up living tissue.

Yet it remains a fact that silicon does indeed form complex chains in rocks and that those chains can easily withstand temperatures up to white heat. Here, however, we are not dealing with chains composed of silicon atoms only (Si-Si-Si-Si-Si) but of chains of silicon atoms alternating with oxygen atoms (Si-O-Si-O-Si). It so happens that each silicon atom can latch on to four oxygen atoms, so you must imagine oxygen atoms attached to each silicon atom above and below, with these oxygen atoms being attached to other silicon atoms also, and so on.

The result is a three-dimensional network, and an extremely stable one. But once you begin with a silicon-oxygen chain, what if the silicon atom's capacity for hooking on to two additional atoms is filled not by more oxygen atoms but by carbon atoms, with, of course, hydrogen atoms attached? Such hybrid molecules, both silicon- and carbon-based, are the 'silicones.'

These, too, have been developed chiefly during World War II and since, and are remarkable for their great stability and inertness. Again, given greater complexity and high temperature, silicones might exhibit the activity and versatility necessary for life.

Another possibility: Perhaps silicones may exist in which the carbon groups have fluorine atoms attached, rather than hydrogen atoms. Fluorosilicones would be the logical name for these, though, as far as I know — and I stand very ready to be corrected — none such have yet been studied.

Might there possibly be silicone or fluorosilicone life forms in which simple forms of this class of compound (which can remain liquid up to high temperatures) might be the background of life and complex forms the principal character? There, then, is my list of life chemistries, spanning the temperature range from near red heat down to near absolute zero: • fluorosilicone in fluorosilicone • fluorocarbon in sulfur • *nucleic acid/protein (O) in water • nucleic acid/protein (N) in ammonia • lipid in methane • lipid in hydrogen Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I've marked it with an asterisk.

This, of course, does not exhaust the imagination, for science-fiction writers have postulated metal beings living on nuclear energy, vaporous beings living in gases, energy beings living in stars, mental beings living in space, indescribable beings living in hyperspace, and so on. It does, however, seem to include the most likely forms that life can take as a purely chemical phenomenon based on the common atoms of the universe. To understand why dwarfs and trolls don't like each other you have to go back a long way.

They get along like chalk and cheese. Very like chalk and cheese, really.

One is organic, the other isn't, and also smells a bit cheesy. Dwarfs make a living by smashing up rocks with valuable minerals in them and the silicon-based lifeform known as trolls are, basically, rocks with valuable minerals in them. In the wild they also spend most of the daylight hours dormant, and that's not a situation a rock containing valuable minerals needs to be in when there are dwarfs around.

And dwarfs hate trolls because, after you've just found an interesting seam of valuable minerals, you don't like rocks that suddenly stand up and tear your arm off because you've just stuck a pick-axe in their ear. Na'ka'leen Feeder, (1994). • Na'ka'leen Feeder, Babylon-5 (1994).

But even if you that away and declare that there are lots of different species of aliens, there is plenty of. Especially in the. Just here on Terra, we can find jellyfish, tarantulas, viruses, and giraffes. Face it, if these fellow Earth-creatures don't resemble us, a totally alien race from another planet. Personally if I open an SF novel only to discover yet another I may need a nausea bag (RocketCat clears his throat then gives me his best 'I'm Looking At A Hypocrite' look). There might be creeping jellies, giant crystals, intelligent plants, mobile fungoids,, dancing in solar coronas, liquid or life, swarming hive intelligences,, and natural 'electronic' life forms in pools of liquid helium.

They might not even be composed of matter as we know it, like the from Dr. Robert Forward's who are made of neutronium and white dwarf star matter. And don't forget the inflatable aliens from John Brunner's. Or the bizarre one from Damon Knight's. Some extraterrestrial creatures inhabit the itself. In Sir Arthur C.

Clarke's was a creature that lived in deep space among asteroid belts. It resembled a huge eye, about twenty feet in diameter. Its survival depended upon the range and resolving power of its eye. Large creatures include the living O'Neil colonies in John Varley's and the from Stanislaw Lem's. Biggest of all is the intelligent nebula from Fred Hoyle's. Well, actually Olaf Stapedon's intelligent galaxies in are bigger, but let's not get carried away.

' Within range of our sensors, there is no life [.]. At least, no life as we know it.' — Spock,, ' These are really alien aliens. They may have: • Non-vertebrate or at least radically. • Nonhuman psychology, as opposed to. • Either unable to survive in Earth-like conditions, or able to survive nearly anywhere. • Vastly different,, or abilities.

•, such as budding, virtual immortality, unfertilized reproduction,, or a. If the aliens in question have two or more of the above traits, you're usually dealing with a Starfish Alien.

However they are still 'people' in the sense of having: • Some kind of language, not necessarily verbal, we can learn to interpret (or, but we can at least recognize it as a language). • Culture • Their own belief systems,.

• A mind-set that admits to things like logic and intuition; not necessarily those things by our definitions, but things like them. • At least some resemblance to living things with which we are familiar. They eat, sleep, reproduce, etc.; they are clearly organic beings, or else.

Sometimes, however, they are too alien and their language, mind-set and culture remain incomprehensible to humans. Often (particularly if the beings can't communicate easily with humans) they will be presumed to be evil by the. But in accordance with, starfish aliens who run across innocent, open-minded humans are themselves known to do beyond-horrible things to them, then excuse themselves later with an explanation that they were only trying to communicate with or greet us in the way they know how. Usually, their language and communication are so different from ours that if there is to be any communication between our species and theirs, it must be done by or them taking on.

Given the long, strange history of life on Earth (a given house includes such a bewildering variety of life as humans, houseplants, pets, spiders, molds, bacteria, etc.), it's likely if we ever actually encounter alien life it might fit in this category. Species that evolve naturally would have adapted to solve similar basic problems: obtaining food/necessities, negotiating natural disaster, adapting to new circumstances, avoiding contamination by pathogens and parasites, competing with other species, competing with themselves, and so forth. So we would expect to find at least a few familiar aspects to their psychology as opposed to sheer indecipherable mystery. If they evolved in similar conditions as us.

These are much more common in animation, video games, and literature than they are in live-action media, due to the likelihood of. They are typically located towards the 'hard' end of the, though when their biology becomes sufficiently improbable, they may soften it instead. When a story is told from the point of view of Starfish Aliens, and other decidedly non human creatures, it's.

The inverse of. Aliens that don't look like humans, but still have basically the same body type are, or, if they're obviously. Effectively split the difference. Prone to enter.

See also,, and. Compare (both tropes have some overlap). The is 's, written in 1931, where the Old Ones are described as 'starfish aliens.' (ed note: see for list of examples). The Elder Things (also known as the Old Ones and Elder Ones) are fictional in the.

The beings first appeared in 's novella, ' (published in 1936, but written in 1931), and later appeared, although not named, in the short story ' (1933). Additional references to the Elder Things appear in Lovecraft's short story ' (1936). Summary Description of a partial headless body: Six feet end to end, three and five-tenths feet central diameter, tapering to one foot at each end. Like a barrel with five bulging ridges in place of staves. Lateral breakages, as of thinnish stalks, are at equator in middle of these ridges. In furrows between ridges are curious growths – combs or wings that fold up and spread out like fans... Which gives almost seven-foot wing spread.

Arrangement reminds one of certain monsters of primal myth, especially fabled Elder Things in the Necronomicon. Lovecraft, At the Mountains of Madness In the Mythos canon, the Elder Things were the first extraterrestrial species to come to the Earth, colonizing the planet about one billion years ago. They stood roughly eight feet tall and had the appearance of a huge, oval-shaped barrel with -like appendages at both ends. The top appendage was a head adorned with five eyes, five eating tubes, and a set of cilia for 'seeing' without light.

The bottom appendage was five-limbed and was used for walking and other forms of locomotion. The beings also had five leathery, fan-like retractable wings and five sets of branching tentacles that sprouted from their torsos. Both their tentacles and the slits housing their folded wings were spaced at regular intervals about their bodies. Lovecraft described the Elder Things as vegetable-like or echinoderm-like in shape, having instead of the of bipeds.

They also differed in that they had a five-lobed brain. The Elder Things exhibited vegetable as well as animal characteristics, and in terms of reproduction, multiplied using spores, although they discouraged increasing their numbers except when colonizing new regions. Though they could make use of both organic and inorganic substances, the Elder Things were carnivorous by preference. They were also amphibious. The bodies of the Elder Things were incredibly tough, capable of withstanding the pressures of the deepest ocean. Few died except by accident or violence. The beings were also capable of hibernating for vast epochs of time.

Nonetheless, unlike many other beings of the Mythos, the Elder Things were made of normal, terrestrial matter. This guide is meant as an aide for the prospective science fiction writer, game designer or world-builder wishing to incorporate extraterrestrial elements, in order to improve quality and rationality of the created works. It is not so much a “ How To”, which would broach multiple sciences and require a profound understanding of each of these, but a “ Before You Go On”, things to consider, wrinkles that need ironing out rather than a methodology. Issues that I bring up here do not necessarily make a choice impossible – you must simply figure out a way around them.

Herein I will be dealing with sapient species, intelligent beings, if you will, since this is where artists’ and writers’ imaginations most often fall short. Here I must distinguish between sentience and sapience – sentient species are aware of their surroundings (which is to say, just about anything more complex than a jellyfish qualifies, even ants), whereas sapient species are capable of reason (humans are the only known organisms that are indisputably sapient). I will do my best to assume a purely physical, rather than cultural or ideological standpoint: alien culture and psychology I may yet examine in the future. Critical Points on Designing Your Sapient: You may have already deduced these points from the former section, but I will reiterate them here.

There are two main things you should have in mind when designing a sapient alien: • A non-sapient “animal” ancestor. • An evolutionary impetus for it to develop sapience. The first of these can be a challenge in its own right, as the non-sapient ancestor must have had some role and adaptation to survive in a particular environment, even if this applies no longer, otherwise it would have never come into existence. This is true of humans as well: even before our brains grew to their modern size, on the plains we specialized as endurance predators, chasing prey till they dropped of exhaustion (and as marathon runners demonstrate, we’re still good at it).

You must imagine where the pre-sapient resided, and how it was capable of surviving in said environment before gaining sapience. I will not outline all the possibilities therein – your imagination will surely outstrip any attempts of mine to list them – but I will provide a series of guidelines and considerations for envisioning it. • Body Plan: it does not serve much purpose to discuss these – the imaginative reader will certainly not be limited by vertebrate-like physiology, or even Earthly physiology. This is for the better, as Earthly biology is not inspirational in this regard – of some forty animal phyla, only two have had major success on land, which is to say there are only two distinct terrestrial body plans – but the less-experienced would do well to study this, particularly non-vertebrate (and even aquatic animal) anatomy, in order to feed their imagination and help them understand the relation of form and function. A complete understanding of your creation’s anatomy is not always necessary, but it is good for determining implications of its various systems – a trachea using life form would not be able to hold its breath, for instance – and you should at least have an idea of how it goes about eating, breathing and reproducing.

That being said, some generalizations of form can be made. • Speed requires sleek, aerodynamic forms, with landrunners possessing long and muscular legs to cross larger distances with every stride: huge numbers of legs do not preclude speed, but managing it does require that they be specially arranged so as to avoid running into each other. • Larger and heavier organisms will opt for columnar legs with few joints (unless they spend most of their time on their bellies or underwater, in which case they have other means of support), while smaller and lighter ones will opt for splayed legs: this is because the former better support weight while the latter can take horizontal forces and moments as might be imposed by wind. • Diggers tend towards compact, cylindrical forms so as to best fit through tunnels, and often minimize or lose their limbs. • Aerial forms specifically adapt themselves to minimize weight, the less that needs to be carried, and typically require energy-rich diets to manage the heavy upkeep of active flight. • Treeclimbers require a means of maintaining grip, which generally implies suction ability or opposable digits, and those that wish to cross from tree to tree without returning to ground will also need good jumping ability or a body part of extensive length to reach across.

• Skeleton: unless your creature is very small (in which case weight is negligible) or lives in a fluid medium (in which case buoyancy counteracts weight), this is a requirement for it to maintain its shape under the load of its own weight, and indeed against other forces that might be arrayed against it. There are fundamentally three kinds of skeleton: exoskeletons (as in arthropods), where the support structure is external and flesh is contained within, endoskeletons (as in vertebrates), where the support structure is internal and flesh is wrapped around it, and mesoskeletons (as in echidnoderms – starfish, crinoids, sea urchins and sea cucumbers), where flesh is both wrapped around the support structure and contained within it. For mechanical reasons, an exoskeleton of a certain mass will always bear the greatest bending stress and be most resistant to buckling, but the external armor carries a heavy price: the rigid armor dulls external sensation, and though it is difficult to penetrate by clawing or biting, it is extremely sensitive to impact loading and is easily shattered by powerful blows. These might be imaginatively compensated for – arthropods have sensitive hairs to feel through their carapace – but exoskeletons are hence presumed to be more viable for small organisms than large ones, as the former do not move fast or far enough to manage such damaging impacts.

Functionally mesoskeletons act much like endoskeletons, albeit the former is somewhat stronger yet with more awkward organ arrangements: these do not provide such armor, but the layers of flesh atop the supports buffer them against impacts. • Diet: chances are your sapient is going to be predatory. Herbivorous sapients are not impossible, as elephants prove, but they’re much less likely to occur than others are for two primary reasons: firstly because plants have far lower energy density than meat and are typically harder to digest, requiring that herbivores spend much more time eating and leaving less time for mental pursuits (elephants eat 18 hours a day), and secondly because such lifestyle does not in and of itself provide the same impetus for intelligence, as it’s not required to secure a meal, whereas carnivores need some ability to outwit and catch their prey. Similar arguments all but preclude the existence of autotrophic sapients, ones that can gather energy without needing to eat at all (most likely by photosynthesis or chemosynthesis): they simply could not gather enough energy by such means to support their activity – a single human being requires as much energy as several thousand tons of grass.

Omnivores stand the best chance, even better than carnivores, as they have the same impetus to develop sapience, but also have fewer limitations on food sources, and hence can more easily substitute when any run out. The technological sapient is under even greater limitations. It must of necessity be social: without regular interactions between individuals, there is no way to transmit information between them, or indeed from generation to generation, and hence no way to accumulate information. You could postulate a species in which the individual inherits information from its parent or acquires it from others biologically, perhaps via genetically encoded memory, but even this would soon be overwhelmed by the increasing efforts required to advance its technology. Only through delegation of effort and resources can continuous achievement be realized: arguably such delegation is the society, or at least its basis. This is not to say that all social species will develop technological capability, but the former is a requirement for the latter: similarly, what is to follow can be applied for non-technological sapients, but cannot be ignored for technological variants. • Communication: society and transmission both require a means of communication, preferably one which can address large groups – any complex species can manage this via physical contact, but this only works on an individual basis.

Barring more exotic means, your public communication must be vision, smell or sound based, and it should go without saying that your sapient must have the required sense be well developed. • Auditory means are already familiar to the reader through human speech, and benefit in that they can transmit information quickly across great distances as well as being difficult to obstruct (particularly infrasound, which can go literally kilometers without much attenuation), but this does not mean that the others are not similarly viable, so long as one takes into account their shortcomings. • Visual displays suffer in that they only work in daytime and line of sight, which makes them easily obstructed: indeed, they can only grab another’s attention if said other is already looking in the right direction (which may not be as big a problem for sapients with panoramic or Omnidirectional vision). Example Design: All these taken together might seem overwhelming, so I’ve provided an example of my own to ease you into the process and demonstrate the contrary.

Crucial to understanding the sapient is understanding its homeworld, the Super-Earth Meios (pictured here ), a terrestrial planet with much higher gravity than the Earth and a surface dominated by ocean, with only the occasional island for relief. One reason for the near landlessness is the soupy atmosphere, which quickly erodes any formations out of existence: volcanic action can outstrip atmospheric destruction for a time, but as soon as the hotspot goes silent, the air will see its works undone. Nevertheless, there are occasions where a number of volcanoes form in near proximity and can hence form a considerably larger landmass that can stand a little longer. It was the formation of such a “subcontinent” that allowed the evolution of chiefly terrestrial life, where before the ecology had been primarily aerial and aquatic, and it is from this picture that our pre-sapient emerged. It was a ballont, member of a clade of organisms that benefitted from the super-dense atmosphere to achieve lighter-than-air flight, and adaptations that formerly suited it for the air were put to good use on land: where their heavier-than-air steelwing competitors had to contend with moving their gravity-enhanced bulk, the ballonts were already able to counteract their weight via buoyancy, the same mechanism that had enabled their flight, and hence could make do without heavyset legs. In particular, it was an ironbelly ballont (as displayed here ), specialized for chasing steelwings down with powerfully beating tails, using its long tentacles to reach through their exoskeleton for energy-rich flesh underneath, and well-armored on its undersides to keep safe from aquatic threats at low altitude – each of these characteristics would be adapted for its new life on land. So long as it stuck close to the ground, there was no longer any threat coming from underneath it, and so the primary danger came from the sky, causing it to flip orientation so that its shell pointed upwards and its balloons earthwards.

Extensions of this would form on the wing-fins and tail, completing its protection, but not solely for this purpose: rather by being semi-rigid as opposed to wholly muscular, these limbs could now push against the ground, allowing them to act as braces against the wind and propel the ballont forward that it may chase down prey with impunity. To this end they took on a sprawling configuration, as they had no need to concern themselves with weight, only inertia (a constant unaffected by gravity). That being said, their ‘feet’ and bottom would remain fleshy, both to feel the earth underneath them as well as to allow better grip and traction.

The success of this body plan lasted only as long as the subcontinent, and when the volcanoes providing for it puttered out one by one, it was only a matter of time before it began to recede. The terrestrial ecosystem was devastated: with their subsistence rapidly disappearing organisms had to return to the water or air or vanish with the landmass. The pre-sapient could not readily do this: while it had maintained the means of flight in its balloons, in adapting its wing-fins and tail for springing it had given up most of its muscles in favor of fewer but stronger units, and without those it could not regain the flexibility and thrust it needed in the air. As the large organisms it once fed off went away with the subcontinent it had to satisfy itself by diversifying its prey, eating everything it could get, and it is in learning how to hunt such numerous prey items without its former speed or grace that it gained sapience. The final design can be found here:. Society had already been present in certain ironbellies before they set foot on the subcontinent, when small groups would chase down and corner packs of smaller fliers, but the basis of it lay in the mother-infant connection.

Because buoyancy requires significant volume, all ballonts give live birth to one or two well-developed young, that they may be born as large and as well-equipped to fly as possible. However, the ironbelly young is born without a shell, that it may better fit inside the mother, and so it is particularly imperative that she defend it: whenever possible she’ll latch her tentacles to those of her young, making sure it’s always within arms’ (tentacles’) reach, and it is from this tentacle-to-tentacle bond that their tactile personal communication is based, while vocal communication is reserved for gaining attention and addressing groups. Other Important Misconceptions: Chances are your sapient does not exist on its own, but as part of a greater science fiction universe, and now you must now consider its place therein – what it thinks of and how it deals with other such races, and what said others think of it and how they deal with it in turn. Ideally this would require understanding the history, culture and psychology of all involved parties, but even ignoring these in favor of solely physical sciences I can caution against certain pitfalls inspired by popular media: • The Single-Biome Planet: barring extraordinary circumstances, few life-bearing planets will fall under this description, and you should not expect your sapient’s homeworld to be one. This is mainly due to two phenomena – the first is variation of temperature with latitude, with areas further away from the equator receiving less sunlight and hence less warmth, and variation of precipitation, brought about by varying temperature, wind direction and topography (with mountains creating rain shadows on their leeward sides) – and is further complicated by the twin effect of atmospheric and oceanic circulation, where fluid currents help to deliver heat across the planet’s surface. You are probably already aware of these, but I’m asking you to apply these lessons: unless your world lacks for ocean or atmosphere, in which case there is nothing to enact the changes of temperature, or these are so prevalent that circulation renders surface conditions all but uniform, your world will have multiple climates.

• Interspecies Romance: I am not talking about platonic relationships – surely we should be able to enjoy the company of a personable sapient – but sexual ones. They will not be prevalent: for the greater part of our population, nonhominid aliens should elicit no sexual response, any more than do animals, plants or inanimate objects to the general observer, but the existence of paraphillia proves that the human form is not necessary for sexual attraction, and presumably, similar pathologies amongst other sapients will allow some to be attracted to those not of their kind.

Not all species will be capable of receptivity – asexual species and some that fertilize externally would have no use for it, and many might only be aroused in designated mating seasons and at no other times – but even amongst those that can, consummating the relationship will be no simple matter. Sexual organs should not be compatible and sexual practice of each species could vary so much between the two as to exclude mutual enjoyment, with some examples possibly posing a danger to one of the partners – many Earthly species have a tendency to devour the male after copulation, and even amongst the comparatively mild mammals and reptiles, quite a few species have spiked penises (including our fellow apes), with the semen of some forming a plug to prevent unequipped males from copulating with claimed females. These might be imaginatively compensated for, but for the most part such relationships will only end in futility. • Interspecies Hybrids: it should go without saying that such species will never be capable of bearing progeny with any other, and none of them, not one, will be able to bear children by us. While a number of interspecies hybrids do exist on Earth, these are only between closely related species, typically within the same genus or family, and many are infertile. Alien sapients would have developed independently, likely with their own unique incompatible biochemistry, and a divergent evolutionary history will ensure that even if the former did match, their genes would not: what chances do they have? Unless the species in question share a common ancestor and are separated by only a short evolutionary period, cross-compatibility simply isn’t possible.

• Interspecies Diet: that is the ability of one race to eat another’s foodstuffs, or indeed any organic matter not from their own world – again, this is unlikely due to divergent biochemistries. It’s not nearly as simple an issue as Mass Effect’s levi versus dextro distinction makes it cut out to be: life forms from different worlds may well be based on the same classes of compounds, yet still find other variants of these expressed by the other to be toxic or indigestible – indeed, all life on Earth is based on proteins, carbohydrates, lipids and nucleic acids, like us, but only a fraction of it is edible. Aliens will find this fraction even smaller, if it exists at all, not even having the benefit of having evolved to eat some of it, though there may be quite a few normally incompatible ‘foods’ that could be processed to yield nutrition.

Suffice to say, with few exceptions sapients will not be sharing foodstuff: each will have to produce and bring along its own specific sustenance. • Interspecies Intelligibility: chances are remote that each species will be able to simulate all the nuances required in the others’ communication, and there’s a good chance that such nuances may even be beyond one’s perception.

This is certainly subjectively true of Earthly languages, with cultures capable of distinguishing phonemes that are synonymous to others, but it’s also objectively true, as we’ve seen in our dealings with the planet’s pre-sapients. On the latter end of the spectrum, elephants and dolphins regularly vocalize with one another, but we only hear the occasional grunt or squeak, in the former case because sound frequency is too low, in the latter case because sound frequency is too high for our ears to pick up. On the former end, apes are certainly capable of perceiving human speech, and with proper training can even comprehend it, but none have yet to vocalize any human words – they simply lack the faculties for it.

This may be imaginatively compensated for – a Russian elephant managed to mime human words by manipulating its lip with its trunk – but for the most part it seems sapients will not be picking up each others’ tongues, and where they do, it will be butchered beyond belief. More likely than not, the two will have to agree to a shared language, or rely on translators. “Two large dark-coloured eyes were regarding me steadfastly. The mass that framed them, the head of the thing, it was rounded, and had, one might say, a face. There was a mouth under the eyes, the lipless brim of which quivered and panted, and dropped saliva. The whole creature heaved and pulsated convulsively.

A lank tentacular appendage gripped the edge of the cylinder, another swayed in the air. There was something fungoid in the oily brown skin, something in the clumsy deliberation of the tedious movements unspeakably nasty.” – H.G. Wells, The War of the Worlds (1898) Pretty disgusting, huh?

The classic tales of science fiction are full of Bug-Eyed Monsters (or BEMs as they are affectionately termed by cognoscenti) which invade planets, threaten towns. Attack rocket ships, and carry off shapely human females.

Hollywood producers apparently are convinced most extraterrestrial (ET) beings fall in one of four zoological categories: (1) Human or humanoid, (2) oversized animals, (3) amorphous blobs and pods, and (4) formless energy beings. Can’t we do any better than this? Anyone with access to a good library can walk in and read all about the biology of one of the most fascinating, richly populated worlds anywhere in the Milky Way: Earth! We inhabit a queer planet with many strange settings and fabulous living creatures, altogether an excellent example of what extraterrestrial life may be all about. To a team of Interstellar Zoologists, researching sentient terrestrial mammals out here in the galactic boondocks, our world is as rare a planetary zoo as any in the Milky Way. Have formulated a simple rule called the Assumption of Mediocrity, which says, in essence, that Earth should be regarded as “typically exotic.” The unusual solutions devised by evolution on this planet to cope with the problem of survival will find their parallels, though not necessarily their duplicates, among the living species of other worlds. As biologist Allen Broms once remarked, “life elsewhere is likely to consist of odd combinations of familiar bits.” Strange Life Life as we know it is based on cells: small, neat packages of living protoplasm containing all of the biological machinery necessary for survival.

Human body cells average a few microns in size. (One micron is a millionth of a meter, about a hundredth of the thickness of the page these words are printed on.) The smallest living thing on Earth capable of independent metabolic activity is the PPLO, or “pleuropneumonia-like organism,” which measures 0.1 microns. Microbiologists estimate that the smallest cell that could, in theory, exist would measure about 0.04 microns in diameter. It is amusing to speculate that the alien analogue to a human being, constructed in the same form but using these miniature cells, would weigh a mere 50 milligrams and stand only 5 millimeters tall – hardly the thickness of a pencil.

Whether creatures so small could retain a human-level intelligence is anyone’s guess. Fairly large extraterrestrial lifeforms might well exhibit acellular physiology, or be unicellular. For example, at one stage in their life history, slime molds are tiny one-celled flagellates capable of individual multiplication by simple fission. In the later “plasmodium” stage of development, large clumps of these creatures fuse together and their cell walls dissolve away to produce an amorphous acellular mass of living protoplasm which can grown as large as 25 centimeters or more.

Further, the largest known single living cell was the egg of the now-extinct half-ton elephant bird or “roc bird” ( Aepyornis maximus). This egg measured about a third of a meter across and weighed 15 kilograms. The number and kinds of organs in alien creatures may also be highly variable. For example, earthly squids have two different kinds of hearts – one for venous and a separate one for arterial blood – and the common earthworm ( Pheretima) has a dozen hearts. Two extinct dinosaur species, Brontosaurus and Diplodocus, had two brains, one in the head and an even larger hunk of neural tissue in the hip region.

(The volume of this “sacral enlargement” in Stegosaurus, another fossil animal of grand proportions, was perhaps twenty times larger than the brain in the cranial cavity! And the entire body of an insect is its “lung” – oxygen is carried directly to cells by an intricate network of tracheae or microtubules permeating the entire organism. Sometimes, organs combine several functions in one – such as the human mouth. ETs need not have the same combinations as we.

They may have identical or separate organs for eating, drinking, excreting, breathing, and speaking. The dolphin, for instance. Eats through its mouth, breathes through its blowhole, and “speaks” through its “ears.” The land snail’s lung opens into a passageway other than its food canal, and sea cucumbers breathe through their rectums (called “anal respiration”). The cloacae of frogs and many other animals is a single organ which combines excretory and reproduction functions. Brachiopods can only vomit excrement from their “blind intestine” (a kind of alimentary cul-de-sac), and the members of phylum Nematomorpha (long worms) eat solely by direct absorption of nutrients through the skin – for they have no mouths. (ed note: This section can be found ) Gravity and Life The respected zoologist D’Arcy Wentworth Thompson once speculated about the.

“Were the force of gravity to be doubled,” Thompson declared, “our bipedal form would be a failure, and the majority of terrestrial animals would resemble short-legged saurians, or else serpents. Birds and insects would suffer likewise, though with some compensation in the increased density of the air. On the other hand, if gravity were halved, we should get a lighter, slenderer, more active type, needing less energy, less heat, less heart, less lungs, less blood. Gravity not only controls the actions but also influences the forms of all save the least of organisms.” It is true that the maximum weight of living species cannot exceed the crushing strength of bony material. But animals are not designed to stand still – if they were, human legs could be a few millimeters thick.

Instead they must bear up under the peak pressures and accelerations encountered during normal running, jumping, and other strenuous survival activities. A horse at rest seems greatly overbuilt; on the racetrack where it may pull to a halt in a second or less, near the breaking point of its bones, the design limits are more fully exploited. Clearly there are other factors at work besides gravitational loading in fixing maximum size – predator/prey relationships, running speeds, food requirements, oxygen levels, ecological constraints, and so forth. Still we can estimate how gravity might influence evolution, based on Earth’s biological history.

The largest land creature alive today is the African elephant, weighing an impressive 6600 kilograms. Tyrannosaurus rex, one of the largest land carnivores, was at least 8000 kg.

The Baluchitherium, the largest extinct land mammal, was built like a hornless rhinoceros, and carried a bulk of more than 22,000 kg. The largest land animal ever may have been Brachiosaurus, of which some specimens may have weighed 111,000 kg. But we’ll ignore this majestic brute because he probably had to spend lots of time sitting in swamps resting his tired bulk. We may conservatively guess that the heaviest exclusively land-dwelling creature plausible on a 1-gee planet is around 22,000 kg. How massive will alien animals be? Of the RAND Corporation and others suggest that terrestrial rocky worlds with atmospheres suitable for life should have surface gravities between about.

Now, if gravity doubles, bone stress won’t increase if a creature’s height is halved while other dimensions remain the same. If maximum height is inversely proportional to gravity, then maximum volume (hence mass) goes inversely as gravity cubed. By this measure the heaviest animal on a 2-gee world is about 2800 kg, while on a 0.2-gee planet (like Saturn’s moon Titan) the most massive beast could conceivably reach nearly three million kilograms – though I’d hate to try to keep it fed! So animals like walruses, small elephants, even 70 kg humanoids are quite possible even on the heaviest of all reasonable Earthlike worlds. No need for “powerfully built, squat creatures, perhaps rather like an armoured pancake on multiple legs. Limited to slow, creeping motions across the surface.” Of course, gravity will affect design.

In any given mass category high-gee animals should have shorter, stockier bones than those evolving in low-gee environments. To provide proper support, bone cross-section must increase directly with weight.

Weight is the product of mass and gravity, so bone diameter must be proportional to the square root of gravity. Let’s apply this to man. The typical human femur, the most perfectly cylindrical and largest single bone in our bodies, measures 3.5 centimeters in diameter. Using the above square-root relation, we find that the thigh-bone should increase to 4.9 cm on a two-gee world or shrink to 1.6 cm on a 0.2-gee planet for identical support of a 70 kg human body mass. Experiments have confirmed that animals reared in high gravity grow thicker bones, stronger hearts, and lose fat, but alien creatures will not appear wildly over- or underbuilt as compared with Earth life of equal mass.

Boneless lifeforms in the sea can grow to enormous sizes. There are other advantages to life without a rigid frame we can hardly appreciate. For instance, an octopus, often called the supreme escape artist, can stretch itself incredibly thin, passing rubberlike through small holes or narrow crevasses and sliding confidently across desktops and the decks of ships. But a creature of land is a denizen of gravity. Surface life must evolve some means of physical support or be reduced to a groveling mass on the ground. On Earth the most common frameworks are the exoskeleton and the endoskeleton. The former, typified by insects and crustaceans, is a hollow bony tube packed with the creature’s viscera.

The latter, which all vertebrates have, is a central spine from which vital organs hang like coats on a hat rack. Exoskeletons are bony material surrounding gut; endoskeletons are bone surrounded by gut. Which design is better? Bioengineers point out that a tubular column always has greater strength than a solid beam of the same mass. Tubes give twice the resistance to bending and many times the opposition to buckling. Mechanical advantages are best exploited by exoskeletons because of the greater bony surface area to which muscles may be attached. So why be vertebrate?

The answer is that we’ve considered only static strength. Large endoskeletons outperform exoskeletons under dynamic impact loading – like falling out of trees – which is why the largest of all animal species have worn their bones on the inside. Massive alien insectoids are not impossible, just less likely. Falling impacts shouldn’t be as severe on low gravity planets, and large active arthropods might survive in a rich oxygen atmosphere.

The greatest carapaced creatures on Earth have ranged in size from a tenth of a meter for the South American tarantula on land up to several meters for certain fossil marine arthropods. ETs have other choices open to them. One of the most popular alternatives among is called the “” found on this planet in marine echinoderms (sea cucumbers. Starfish, sea urchins) and the cormorant (a seabird of the pelican family).

Physical stress passes through the body along a kind of bony trellis, an unusual internal arrangement which one wag has facetiously termed “bowels in a birdcage.” Another possibility is the double spine or multiple endoskeleton. On Earth flatworms and other free-living turbellarians have twin neural channels running the length of their bodies. Alien “ladder skeletons” might improve postural stability and provide greater strength on high-gravity worlds, though turning or twisting motions of the trunk might be restricted even if the multiple support posts are jointed or segmented. A third alternative is the “hydrostatic skeleton,” surprisingly common on Earth. Animal bodies are kept stiff by pressurized fluid trapped in a sack of tough skin. Mostly only small earthworms and nematodes have this support, but massive sea creatures such as sharks compress their innards to help negotiate sharp turns and even man uses the contents of his abdomen as a hydrostatic skeleton.

Large aliens might evolve a liquid skeleton inside taut, fiber-strengthened tubes with extensive reinforcing musculature – purely hydrostatic caterpillars, for example, have about 4000 individual muscles as compared to less than 700 for a human being. How Many Eyes? Nature often uses the same solution to a given problem encountered by many independently evolved species.

Perhaps one of the most striking instances of this “convergent evolution” is the “camera eye’’ invented separately by at least five major terrestrial animal phyla (chordates, mollusks, annelids, coelenterates and protists). Each have radically different developmental histories. Naturally there are a few discrepancies – for example, light-sensitive cells in molluscan eyeballs point towards the light, the opposite of vertebrates. But the adjustable lens. Retina, pigments, focusing muscles, iris diaphragm, transparent cornea and eyelids all are immediately recognizable. Nature is perhaps trying to tell us something: The camera eye is ubiquitous because it’s simply the best design for the job, on this or any other world. Combat Patch For Bosnia Sfor 5.

The next most successful – indeed more so if you just count species – is the compound eye of insects and crustaceans. Each organ looks like a small multifaceted jewel, actually a tiny bundle of optical tubes that direct light onto a large matrix of individual photosensitive spots on the retina. The image forms a composite mosaic of thousands of little light-dots. (Dragonfly eyes have more than 28,000 facets and can discern motion up to twelve meters away.) The compound eye, however, has such poor resolving power that an insect poring over this page of print would be quite unable to make out the individual letters, so large ETs will find the system unattractive.

It seems best for smaller creatures – if a flea had a spherical lens eyeball like that of humans, the pupil would be so minute that diffraction effects would utterly ruin the image. Other visual techniques of limited importance on Earth may be emphasized on other planets. For instance, alien species may have “pinhole camera” eyes like the chambered nautilus, a beautifully simple system consisting of an open optical pit without lenses, exceptionally useful in water. In the “scanning eye” of the snail, light penetrates a simple crystalline lens and is scanned by a single retinal nerve sensor moving across the visual field, slowly building up an image of the environment. The principle of the optical reflector telescope has never been developed for direct imaging on this world, though many species use a biological mirror assembly to increase camera eye sensitivity (the tapetum of the common tabby cat) or to attract prey using deep-sea “searchlights” in conjunction with bioluminescence (the retractable reflectors of the luminous squid). How many eyes are best? Nature usually economizes, so a single receptor organ is good enough for nondirectional sensing.

Most large organisms have but one organ of smell and one of taste. On the other hand, directional senses can make good use of the benefits of stereo. Triangulation and depth perception require at least two physically separated receptors, and there seems little to be gained by going to more than a single pair. As astronomer Carl Sagan once pointed out, “Three eyes represent not nearly the same improvement over-two that two represent over one.” Nevertheless a few animal species do have more than one pair of imaging eyes. Zoologist Norman J. Berrill of McGill University in Montreal describes the dinnertime antics of the spider, which has four pairs of eyes: “The rear pair serve to watch behind for either food or danger. The other three pairs work together but in succession.

If something comes within the range of vision of one of the outermost pair, the head turns until the object is brought into the field of the two pairs of eyes in the middle, and the spider then advances. When the object is brought into focus of the forward pair, the spider jumps to attack.” The ultimate limit is probably reached by the scallop, whose literally hundreds of tiny, beautifully constructed nonimaging “eyes” are spread around the circumference of its mantle like running lights on an ocean liner. What about eyes on stalks? Most regard this as a rather unlikely adaptation for thinking animals.

Eyestalks require a hydraulic support system inefficient except in small animals. Eyes are vital senses for large organisms, yet stalks could be lopped off by predators with a single stroke of claw or pincer, permanently depriving the owner of sight. Periscoping eyes unprotected by bone are also more prone to common injury – in an accident, stalks could be bumped, slammed or squashed all too easily. Alien Senses, of course, is simply the detection of one narrow set of wavelengths of light within the entire electromagnetic spectrum.

One alternative to “visual” sight is, or seeing with heat waves. The rattlesnake is quite good at this – the creature has two imaging eyeballs operating in the visible, and two conical pits on either side of the head which permit binocular IR sensing of temperature differences as little as 0.002 °C. The theory of optics predicts that alien infrared eyeballs with resolution close to that of the human eye could have apertures as small as 4 centimeters at 93,000 Angstroms (the peak wavelength of black body radiation emitted by a warm human body). This compares well with the size of the eye of the Indian elephant (4.1 cm), the horse (5 cm), the blue whale (14.5 cm), and the largest cephalopods (up to 37 cm).

Is another possibility, although there are two major evolutionary problems with this. First, it is difficult (though not impossible) to imagine planetary surface conditions in which the illumination in the radio band is equal to or greater than the brightness in the visible, thus giving radio vision the competitive edge. Second, radio sensors would have to be on the order of 10-1000 meters wide to achieve human-eye acuity, though this resolution may not be absolutely necessary. Assuming life evolves primarily on planetary surfaces and under air, other forms of vision – very low frequency, ultraviolet, and x-ray – are unlikely because these wavelengths are strongly absorbed during the passage through atmosphere or ocean. Static electric field sensing has been documented in numerous species, notably sharks and electric fishes, and sensitivity to magnetic fields has been found in snails, pigeons; dolphins, bees.

And many other animals. The acoustical, tactile, and chemical spectra of sensation have also been well exploited by life on Earth.

One possible extraterrestrial sense often overlooked is the ability to detect radioactivity. On a world with highly concentrated radionuclide ores near the surface, or on a planet in the throes of a global nuclear holocaust, biological Geiger counters would give warning to steer clear of large tracts of radiation hazards. The “radioactive sense” was once artificially bestowed on a small group of laboratory animals by wiring portable Geiger counters directly to the fear center of feline brains.

When confronted with a pile of radioactive materials in one comer of their cages, each cat shied away. The key to alien senses is survival – any environmental information that would permit an animal to better compete for the limited resources available is a valid candidate for sensing. For example, we could imagine a sophisticated meteorological sensorium evolving on a world cursed with highly volatile, perpetually inclement weather.

Humidity and barometric sensors would be essential, as would anemometers to calibrate wind velocity. The ability to sense changes in atmospheric composition, such as the carbon dioxide detectors possessed by honeybees and fire ants, would be useful. Atmospheric turbidity, closely related to developing weather patterns, greatly influences the degree of skylight polarization – sensors responsive to the intensity and distribution of polarized light might permit their owner to seek shelter from the elements before disaster struck.

The seeming ability of many animals to sense an earthquake or tornado before it arrives may relate to their perception of very low frequency infrasonics or minute electrical field variations immediately preceding the event. And the allegation that elephants can sense water located a meter or so beneath the surface of apparently dry riverbeds is unproven scientifically, yet the fact remains that such biological dowsers would be tar more likely to survive on a drought-stricken planet. On strictly mechanical grounds, three points are needed geometrically to define a surface plane – two points make only a line.

ETs trying to stand up on just one or two levers will promptly fall flat on their faces. We bipedal humans manage to remain erect only because our large feet provide additional points of contact with the ground, but without toes or feet a minimum of three legs is necessary. Are tripedal aliens possible?

Traditional biologists say no. A walking three-legger must lift at least one limb off the ground, at which instant it loses its planar support base, a situation statically unstable and dynamically precarious. Four legs seem better from an engineering point of view, as the creature can remain balanced when a leg is in motion. Ancestral fishes only have fins in pairs, so mustn’t all limbs evolve in pairs as well? Remain unconvinced. Most running bipeds and quadrupeds keep two or fewer limbs on the ground during locomotion, so three-point dynamic stability is probably unnecessary. Land life need not always evolve from pair-finned fishes – descendants of, say, a starfish might be odd-leggers.

Most persuasive, however, is the simple fact that tripeds exist on Earth! The extinct Tyrannosaurus rex and a few large contemporary creatures such as the kangaroo run bipedally but stand tripedally.

The tails of these animals are as strong and thick as the forelegs and are regularly used for postural support. Indeed, when kangaroos fight, they rear up on their tails, freeing both legs to deliver crushing kicks to opponents.

More legs than four are plausible even for massive, intelligent animals. Odd appendages are often used for highly specialized purposes, as witness the prehensile tail of monkeys and the dexterous trunk of elephants. The key to higher multipedia is neural control. The nervous circuitry for an extra limb is far less than that required to add, say, another eye. Muscles need thousands of new neurons, but eyeballs need millions. About one-third of the mammalian brain is committed to sensory functions, whereas only a small slice handles motor control, ETs are much more likely to have extra arms than extra eyes or ears. Bonnie Dalzell, a writer-paleontologist who helped Larry Niven work out some of his fictional aliens, insists that vertebrates on Earth have four limbs solely because of the common descent from fishes adapted to free-swimming conditions in large open oceans.

These fish needed only two independent sets of diving planes to make a go of it in the sea. Perhaps if we evolved instead from Euthacanthus, a Devonian Period fish boasting no fewer than seven pairs of fins, we might be hexapodal or more-podal today ourselves. Dalzell expects to find intelligent six-leggers on worlds with small, shallow oceans.

There, bottom-dwelling fishes would become the predominant coastal and freshwater lifeforms early in evolutionary history. If the planet has a very seasonal climate, perhaps accompanied by large-scale periodic evaporations of lakes and seas, few fish species could evolve into good swimmers as on Earth. Marine creatures with many pairs of fins would have the advantage, ultimately inheriting the land and producing a rich ecology of multipodal animal life. There are many advantages to six-legged living. On high-gravity worlds hexapedia is a good way to distribute mechanical stresses and help reduce the danger of bone breakage. Injury or loss of a limb is more catastrophic for four-leggers than for six-leggers (who have “spares”). Hexapods also have better balance since, unlike quadrupeds, they can keep a stable support tripod on the ground even when running at high speeds.

And it shouldn’t be too hard to coordinate all those legs. Says Dalzell: “Earthly insects with three pairs of legs are hardly noted for their well-developed mental powers, but most of them walk just fine.” Of course, legs are not the only game in town. The potential of rotary motion (to pick one possibility of many) cries out for fulfillment. A few years ago biologists made the amazing discovery that the tails of tiny bacteria are driven by minute ionic motors complete with rotors, stators, bushings and freely-rotating drive shafts spinning up to 60 cycles per second. The rapid back-and-forth wiggling of flagella we see under the microscope is actually a complicated helical twisting movement more akin to a propeller screw than to a simple fishy undulation. This finding contradicts the long-standing dictum that living organisms may not contain detached, self-rotating parts. Rotary motion may be possible for large animals too.

Picture a small Earthlike world with little tectonic activity and broad, flat continental shelves flooded to a depth of five or ten meters during global warm spells. A creature not unlike the molluscan cuttlefish Sepia hovers near the bottom, stalking small fish, shrimps, and crabs, sometimes jetting about by expelling water rapidly from several exit portals like many other cephalopods. Occasionally sand particles jam in a portal, causing irritation. The animal responds by encasing them in a perfectly smooth spherical pearl, much like those of the modem oyster. Millions of years later an Ice Age arrives. The retreating shoreline leaves behind vast tracts of smooth hard continental shelf. Forced into ever more turbid, colder, shallower waters, we might imagine our cuttlefish eventually abandoning the sea for land, evolving into a “caster creature.” Its jet ports now permanently plugged by large pearly structures almost from birth, these animals might develop the ability to roll along the graded continental raceways.

Speed is controlled by internal sphincters aided by heat sensors for guided braking on gentle downhill stretches and a “low-gear” muscular assist for steep climbs. Tentacle arms like ski poles provide additional stability on fast runs along the coastline. On Earth the albatross is pretty close to the maximum. This 10 kilogram bird reaches wingspans up to four meters and needs a lengthy runway to achieve takeoff speed of 20 kph. This minimum velocity is called the “stall speed” and is partly determined by air density. Venusian pigeons could remain airborne at speeds ten times slower than their Earthly cousins, whereas Martian birds of similar size and shape would have to fly ten times faster to stay aloft. The main factor fixing avian size is atmospheric pressure, not gravity as some erroneously believe.

On high-pressure worlds, alien bird creatures can have surprisingly small wings and large masses. An extraterrestrial with the mass of a man could fly with the wings of an albatross in air just five times thicker than Earth’s, and a Venusian albatross could make do with stubby wings smaller than the page on which these words are printed. Planetary surface gravity has less effect on size in part because it varies far less than air density from world to world. For the same ease of flight a pigeon on a 2-gee planet with Earthlike air must increase total wing area by only 75 percent; on a bantam-weight 0.2-gee world, wing surface may decrease 75 percent. Gravity also influences stall speed. An albatross on a 2-gee planet needs a 40-percent runway extension; on a 0.2-gee world it can get by with 55 percent less.

Massive extraterrestrial avians are more likely on puny planets with dense atmospheres. How many wings are best?

Most common among terrestrial species is a single pair which generate lift by actively beating the air something like the blades of a helicopter rotor. Less common is the “airplane” system, with one pair producing passive lift (like the wings of an airplane) and a second pair taking the more active role (like propellers).

Adding yet more wings would serve no useful purpose, hence are unlikely to evolve. Only a very few insect species on Earth retain vestigial traces of an ancestral third wing pair, and these are degenerate and useless for flight.

Alien air travelers may have no wings at all! There are many alternatives that have never been fully exploited by evolution on this planet. Consider, for example, the principles of the rocket, the glider, and the balloon. A high-gravity world with abundant seas and a warm, thick oxygen atmosphere might produce a “rocket fish” predator, patrolling the coastal shallows and preying on bird-sized torpid insect life thickly swarming high up. Much like the toy plastic projectiles that shoot the length of a playing field when fully charged with water and compressed air, the rocket fish bolts from the sea skyward and mouthes its dinner on the fly. Such an animal must have a sturdy posterior pressure canister that can be discharged rapidly through a rigid bony nozzle, rechargeable in minutes using powerful sphincter muscles, internal gas generation, or osmosis. Earthly precedents include the jet propulsion of squids and octopuses, the pressurized chemical sprays of warrior termites, and the boiling liquid jet of the bombardier beetle.

A lightweight planet with high winds might be ideal for the evolution of sentient “parachute beasts,” large aerial aliens able to navigate the airways of their world by manipulating sturdy chutes or simple gliding surfaces. Vultures can sail for hours with little effort using strong mountain updrafts to gain altitude, but other worlds may be even better suited for this mode of flight.

Further terrestrial precedent includes the aerial dispersal of spider young – spiderlings crawl to the tip of a blade of grass, raise their tiny abdomens and let fly a thin silken thread, then hop aboard as a gust of wind catches the gossamer strands and whisks them away into the sky. The idea of balloonlike living organisms is an old one both in science and science fiction. Bonnie Dalzell designed an “airship beast” for the Pick-a-Planet exhibit at the Smithsonian’s National Air and Space Museum. These creatures supposedly inhabit a world with cold winters, heavy gravity and a thick atmosphere.

Twice a year the herbivorous hundred-kilogram blimps inflate their many lifting bags with metabolically generated hydrogen gas and drift to the opposite hemisphere to avoid the seasonal chill. Strong winds are an advantage, but predators are numerous and many noble aeronauts are lost during the migrations when a chance bolt of lightning strikes and ignites their flammable bodies. On Earth the Portuguese man-of-war, the chambered nautilus, and swim bladders in fishes provide precedent for a balloon lifestyle in a fluid medium. Sail power has also been largely neglected in biology for animal locomotion. One of the few examples on this planet is Velella, a small, baggy, disk-shaped sea creature whose sail-like dorsal fin permits it to drift slowly with the wind. Another example is, surprisingly, the whale.

These majestic cetaceans sometimes “stand on their heads” exposing only their giant broadleaf tails above water, catching gusts of wind and playfully “sailing” for hundreds of meters before coming up for air. More than forty years ago Olaf Stapledon speculated on the possibility of a true biological sailboat. Let us imagine a cephalopod with a heavy concave shell living in the bays and estuaries of some alien world. Over the years the species gradually acquires the ability to float boatlike on the inverted shell as an aid in migration.

These creatures drift with the shore currents, feeding on surface algae and nibbling the tops of seaweed stalks. In time the shell could become better adapted for navigation, perhaps with a streamlined undercarriage, allowing the ET to better chart its course between known patches of food and to escape its predators. Eventually it gains still more speed with a crude sail, a thin membrane growing from a shank of cartilage in the animal’s belly. With further evolution the membrane becomes retractable, even delicately manipulatable by fine muscles. At last the emergence of a brain and sensory organs strictly comparable to those of higher mollusks on Earth makes possible a kind of living clipper ship complete with masthead (forward sensors), jib, mainsail, riggings (extensible tendon), and a rudder. Every habitable planet has millions of living species and billions of extinct ones, and there are many trillions of useful planets in the universe. This adds up to an incredible diversity of life.

Christian Huygens wrote in The Celestial Worlds Discover’d (1698) that “Nature seems to court variety in her Works, and may have made them widely different from ours either in their matter or manner of Growth, in their outward Shape, or in their inward Contexture; she may have made them such as neither our Understanding nor Imagination can conceive.” Whether Huygens’s prophecy is true is something we can determine only by traveling to faraway worlds and sampling extraterrestrial ecologies at close hand. Perhaps, someday soon, we will make this epic journey. FOR FURTHER READING Norman J. Berrill, Worlds Without End: A Reflection on Planets, Life and Time, Macmillan, New York, 1964. McNeill Alexander, G. Goldspink, eds., Mechanics and Energetics of Animal Locomotion, John Wiley & Sons, New York, 1977. Bonnie Dalzell, “Exotic Bestiary for Vicarious Space Voyagers,” Smithsonian 5 (October 1974):84-91.

Doris Jonas, David Jonas, Other Senses, Other Worlds, Stein and Day, New York, 1976. Freitas Jr., “Xenobiology,” Analog 101(30 March 1981):30-41. Olaf Stapledon, Star Maker, Methuen, 1937. Reprinted: Penguin Books, Baltimore, Maryland, 1972. NOTES TOWARD A DEFINITION OF RELATEDNESS Before space flight it was often predicted that other planets would appeal strictly to the intellect. Even on Earthlike worlds, the course of biochemical evolution must be so different from the Terrestrial—since chance would determine which of many possible pathways was taken—that men could not live without special equipment. And as for intelligent beings, were we not arrogant to imagine that they would be so akin to us psychologically and culturally that we would find any common ground with them?

The findings of the earliest extra-Solar expeditions seemed to confirm science in this abnegation of anthropomorphism. Today the popular impression has swung to the opposite pole.

We realize the galaxy is full of planets which, however exotic in detail, are as hospitable to us as ever Earth was. And we have all met beings who, no matter how unhuman their appearance, talk and act like one of our stereotypes. The Warrior, the Philosopher, the Merchant, the Old Space Ranger, we know in a hundred variant fleshly garments. We do business, quarrel, explore, and seek amusement with them as we might with any of our own breed. So is there not something fundamental in the pattern of Terrestrial biology and in Technic civilization itself? As usual, the truth lies somewhere between the extremes. The vast majority of planets are in fact lethal environments for man.

But on this account we normally pass them by, and so they do not obtrude very much on our awareness. Of those which possess free oxygen and liquid water, more than half are useless, or deadly, to us, for one reason or another. Yet evolution is not a random process. Natural selection, operating within the constraints of physical law, gives it a certain direction. Furthermore, so huge is the galaxy that the random variations which do occur closely duplicate each other on millions of worlds. Thus we have no lack of New Earths. Likewise with the psychology of intelligent species.

Most sophonts indeed possess basic instincts which diverge more or less from man's. With those of radically alien motivations we have little contact. Those we encounter on a regular basis are necessarily those whose bent is akin to ours; and again, given billions of planets, this bent is sure to be found among millions of races. Of course, we should not be misled by superficial resemblances. The nonhuman remains nonhuman. He can only show us those facets of himself which we can understand. Thus he often seems to be a two-dimensional, even comic personality.

But remember, we have the corresponding effect on him. It is just as well that the average human does not know on how many planets he is the standard subject of the bawdy joke.

Even so, most races have at least as much contrast between individuals—not to mention cultures—as Homo Sapiens does. Hence there is a degree of overlap. Often a man gets along better with some nonhuman being than he does with many of his fellowmen. 'Sure,' said a prospector on Quetzalcoatl, speaking of his partner, 'he looks like a cross between a cabbage and a derrick. Sure, he belches H 2S and sleeps in a mud wallow, and his idea of fun is to spend six straight hours discussin' the whichness of the wherefore. But I can trust him—hell, I'd even leave him alone with my wife!'

'As I remember,' he said, collecting his thoughts rapidly, 'the biologists asked themselves the question, 'If we had no preconceived ideas, and were starting with a blank sheet of paper-how would we design an intelligent organism?' ' 'I'm not much of an artist,' Floyd apologized, after he had managed to borrow paper and pencil, 'but the general conclusion was something like this.' He sketched quickly, and when he had finished Mr. Kelly said, 'Ugh!' 'Well,' chuckled Floyd, 'beauty lies in the eye of the beholder. And talking of eyes, there would be four of them, to provide all-round vision.

They have to be at the highest part of the body, for good visibility—so.' He had drawn an egg-shaped torso surmounted by a small, conical head that was fused into it with no trace of a neck. Roughly sketched arms and legs were affixed at the usual places.

'Getting rid of the neck removes a fundamental weakness, we only need it because our eyes have a limited field of view, and we have to turn our heads to compensate.' 'Why not a fifth eye on top, for upward vision?'

Asked Kaminski, in a tone of voice which showed what he thought of the whole concept. 'Too vulnerable to falling objects. As it is, the four eyes would be recessed, and the head would probably be covered with a hard protective layer. For the brain would be somewhere in this general region—you want the shortest possible nerve connections to the eyes, because they are the most important sense organs.' 'Can you be sure of that?' 'No—but it seems probable.

Light is the fastest, longest-ranging carrier of information. Any sentient creature would surely take advantage of it. On our planet, eyes have evolved quite independently, over and over again, in completely separate species, and the end results have been almost identical.' 'I agree,' said Whitehead.

'Look at the eye of an octopus—it's uncannily human. Yet we aren't even remote cousins.' 'But where's the thing's nose and mouth?' 'Ah,' said Floyd mischievously, 'that was one of the most interesting conclusions of the study.

It pointed out the utter absurdity of our present arrangements. Fancy combining gullet and windpipe in one tube and then running that through the narrow flexible column of the neck!

It's a marvel we don't all choke to death every time we eat or drink, since food and air go down the same way.' Kelly, who had been sipping at a highball, rather hastily put it down on the buffet table behind her. 'The oxygen and food intakes should be quite independent, and in the logical places. Floyd sketched in what appeared to be, from their position, two oversized nipples. 'The nostrils,' he explained.

'Where you want them—beside the lungs. There would be at least two, well apart for safety.' 'And the mouth?' 'Obviously—at the front door of the stomach. The ellipse that Floyd sketched was too big to be a navel, though it was in the right place, and he quickly destroyed any lingering resemblance by insetting it with teeth. 'As a matter of fact,' he added, 'I doubt if a really advanced creature would have teeth. We're rapidly losing ours, and it's much too primitive to waste energy grinding and tearing tissues when we have machines that will do the job more efficiently.'

At this point, the Vice-President unobtrusively abandoned the canape he had been nibbling with relish. 'No,' continued Floyd remorselessly.

'Their food intake would probably be entirely liquid, and their whole digestive apparatus far more efficient and compact than our primitive plumbing.' 'I'm much too terrified to ask,' said Vice-President Kelly, 'how they would reproduce. But I'm relieved to see that you've given them two arms and legs, just like us.'

'Well, from an engineering viewpoint it is quite hard to make a major improvement here. Too many limbs get in each other's way; tentacles aren't much good for precision work, though they might be a useful extra. Even five fingers seems about the optimum number; I suspect that hands will look very much the same throughout the universe even if nothing else does.' 'And I suspect,' said Kaminski, 'that the people who designed our friend here failed to think far enough ahead. What's the purpose of food and oxygen?

Why, merely combustion, to produce energy-at a miserable few percent efficiency. This is what our really advanced extraterrestrial will look like. He took the pen and pad from Floyd, and rapidly shaded the egg-shaped body until the air and food intakes were no longer visible. Then, at waist level, he sketched in an electric power point-and ran a long cable to a socket a few feet away. There was general laughter, in which Kaminski did not join, though his eyes twinkled. 'The cyborg-the electromechanical organism. And even he-it is only a stepping stone to the next stage-the purely electronic intelligence, with no flesh and-blood body at all.

The robot, if you like-though I prefer to call it the autonomous computer.' Conditions are so difierent on Mars and—to our earth-centered feelings—so inferior from those on earth that scientists are confident no intelligent life exists there. If life on Mars exists at all (the probability of which is small, but not zero) it probably resembles only the simplest and most primitive terrestrial plant life. Still, even granted that the likelihood of complex life is virtually nonexistent; we can still play games and let our fancy roam.

Let us suppose that we are told flatly: “There is intelligent life on Mars, roughly man-shaped in form.” What reasonable picture can we draw on the basis of what we now know of Mars—bearing always in mind that the conclusions We reach are not to be taken seriously, but only as an exercise in fantasy? In the first place, Mars is a small world with a gravitational force only two-fifths that of earth. If the Martian is a boned creature, those bones can be considerably slenderer than ours and still support a similar mass of material (an inevitable mechanical consequence of decreased weight). Therefore, even if the torso itself were of human bulk, the legs and arms of the Martian would seem grotesquely thin to us. Objects fall more slowly in a weak gravitational field and thus the Martians could afford to have slower reflexes. Therefore, they would seem rather slow and sleepy to us (and they might be longer-lived because of their less intense fight with gravitation). Since things are less top-heavy in a low-gravity world, the Martian would probably be taller than earth people.

The Martian backbone need not be so rigid as ours and might have two or three elbowlike joints, making stooping from his (possible) eight-foot height more convenient. The Martian surface has been revealed by the Mars-probe, Mariner IV, to be heavily pockmarked with craters, but the irregularities they introduce are probably not marked to a creature on the surface. Between and Within the craters, much of the surface is probably sandy desert. Yellow clouds obscuring the surface are occasionally detected and, in the 1920s, the astronomer E. Antoniadi interpreted these as dust storms.

To travel over shifting sands, the Martian foot (like that of the earthly camel) would have to be flat and broad. That type of foot, plus the weak gravity, would keep him from sinking into the sand. As a guess, the feet might be essentially triangular, with three toes set at 120° separation, with webbing between. (No earthly species has any such arrangement, but it is not an impossible one. Extinct flying reptiles, suchlas the pterodactyl, possessed Wings formed out of webbing extending from a single line of bones.) The hands would have the same tripod development, each consisting of three long fingers, equally spaced. If the slender finger bones were numerous, the Martian finger would be the equivalent of a short tentacle.

Each might end in a blunt swelling (like that of the earthly lizard called the gecko), where a rich network of nerve endings, as in human fingertips, would make it an excellent organ for touching. The Martian day and night are about as long as our own, but Mars is half again as far from the sun as we are, and it lacks oceans and a thick atmosphere to serve as heat reservoirs. The Martian surface temperature therefore varies from an occasional 90° Fahrenheit, at the equatorial noon, down to a couple of hundred degrees below zero, by the end of the frigid night. The Martian would require an insulating coating. Such insulation might be possible with a double skin; the outer one, tough. Horny, and water impervious, like that of an earthly reptile; the inner one, soft, pliable, and richly set with blood vessels, like that of an earthly man. Between the two skins would be an air space which the Martian could inflate or deflate.

At night the air space would be full and the Martian would appear balloonlike: The trapped air would serve as an insulator, protecting the warmth of the body proper. In the warm daytime, the Martian would deflate, making it easier for his body to lose heat. During deflation, the outer skin would come together in neat, vertical accordian pleats.

The Martian atmosphere, according to Mariner IV data, is extremely thin, perhaps a hundredth the density of our own and consisting almost entirely of carbon dioxide. Thus, the Martian will not breathe and will not have a nose, though he will have a strongly muscled slit—in his neck, perhaps— through which he can pump up or deflate the air space. What oxygen he requires for building his tissue structure must be obtained from the food he eats. It will take energy to obtain that oxygen, and the energy supply for this and other purposes may come directly from the sun. We can picture each Martian equipped with a capelike extension of tissue attached, perhaps to the backbone.

Ordinarily, this would be folded close to the body and so would be inconspicuous. During the day, however, the Martian may spend some hours in sunlight (clouds are infrequent in the thin, dry Martian air) with his cape fully expanded, and resembling a pair of thin, membranous wings reaching several feet to either side. Its rich supply of blood vessels will be exposed to the ultraviolet rays of the sun, and these will be absorbed through the thin, translucent skin.

The energy so gained can then be used during the night to enable the necessary chemical reactions to proceed in his body. Although the sun is at a great distance from Mars, the Martian atmosphere is too thin to absorb much of its ultraviolet, so that the Martian will receive more of these rays than we do.

His eyes will be adapted to this, and his chief pair, centered in his face, will be small and slitlike to prevent too much radiation from entering. We can guess at two eyes in front, as in the human being, since two are necessary for stereoscopic vision—a very handy thing to have for estimating distance. It is very likely that the Martian will also be adapted to underground existence, for conditions are much more equable underground. One might expect therefore that the Martian would also have two large eyes set on either side of his head, for seeing by feeble illumination.

Their function would be chiefly to detect light, not to estimate distance, so they can be set at opposite sides of the head, like those of an earthly dolphin (also an intelligent creature) and stereoscopic vision in feeble light can be sacrificed. These eyes might even be sensitive to the infrared so that Martians can see each other by the heat they radiate. These dim-vision eyes would be enormous enough to make the Martian face wider than it is long. In daytime, of course, they would be tightly closed behind tough-skinned lids and would appear as rounded bulges. The thin atmosphere carries sound poorly, and if the Martian is to take advantage of the sense of hearing, he will have to have large, flaring, trumpetlike ears, rather like those of a jackrabbit, but capable of independent motion, of flaring open and furling shut (during sandstorms, for instance). Exposed portions of the body, such as the arms, legs, ears, and even portions of the face which are not protected by the outer skin and the airtrap within, could be feathered for warmth in the night. The food of the Martian would consist chiefly of simple plant life, which would be tough and hardy and which might incorporate silicon compounds in its structure so that it would be gritty indeed.

The earthly horse has teeth with elaborate grinding surfaces to handle coarse, gritty grass, but the Martian would have to carry this to a further extreme. The Martian mouth, therefore, might contain siliceous plates behind a rounded opening which could expand and contract like a diaphragm of a camera. Those plates would work almost like a ball mill, grinding up the tough plants. Water is the great need.

The entire eater supply on Mars is equal only to that contained in Lake Erie, according to an estimate cited by astronomer Robert S. Consequently, the Martian would hoard the water he consumes, never eliminating it as perspiration or wastes, for instance. Wastes would appear in absolutely dry form and would be delivered perhaps in the consistency, even something of the chemical makeup, of earthly bricks.

The Martian blood would not be used to carry oxygen, and would contain no oxygen-absorbing compound, a type of substance which in earthly creatures is almost invariably strongly colored. Martian blood, therefore, would be colorless. Thus the Martian skin, adapted to ultraviolet and absorbing it as an energy source, would not have to contain pigment to ward it off.

The Martian therefore would be creamy in color. The extensible light-absorbing cape, particularly designed for ultraviolet absorption, might reflect longwave visible light as useless. This reflected light could be yellowish in color.

This would cause our Martian to seem to be (when he was busily absorbing energy from solar radiation) a dazzling white creature with golden wings and occasional feathers. So ends our speculation—in a vision of Martian forms not so far removed from the earthman’s fantasies of the look of angels.

For at least the last three decades, a large number of science fiction writers have been confronted, at one time or another, with the problem of constructing extraterrestrial lifeforms. Naturally the professional chemists and biologists who write science fiction on the side did best, not so much because their professional knowledge led them for long distances on hitherto untrodden paths, but because it made them stop at the right moment.

As regards those who were primarily writers, the results make one suspect that they at first tried to apply what biology they knew. Since this apparently did not get them very far, they presumably threw overboard whatever it was they had not quite arrived at and wrote things like this: “Surprisingly, the aliens were quite human in shape, the only major differences, or at any event the ones which were easily visible, being a strong tail and a bluish complexion.” Or else, if the actual contact with the aliens could be fleeting, they resorted to saying that the forms the Earthmen beheld were so alien, so outside of all terrestrial experience, that it was impossible to describe them.

However, the occasional science fiction writer of the past was not the only type of creative genius who did, or could have, exerted ingenuity in the building of an extraterrestrial. There were many others who engaged in a very similar line of endeavor for the purpose of representing gods, demons or just outlandish creatures, somewhat along the line of the Midnight Marvels to which I devoted a column some months ago. To put it bluntly, nobody showed much imagination and the method was standardized at an early age: Combine the features of various kinds of living creatures into something that could be drawn, painted or sculptured and the job was done. Put a woman’s head on a feline body and you had a sphinx. Add the head of a bird to the body of a man and you had ibis-headed Thoth. Take a horse and supply it with the wings of an eagle and Pegasus was ready for flight, though with lateral stability only.

Take another horse, cut off its head and graft the upper half of a man’s body to it and the centaur was ready. So you obviously cannot produce a biologically possible or even believable creature by the (random or artistic) combination of separate parts. Fine — but how can you go about it? All I can say offhand is that it isn’t easy; so much depends on so many different circumstances.

There is, in the first place, the planetary environment, consisting of such factors as either much water or very little water; temperature which depends mainly but not only on the distance of the planet from its sun; seasonal changes which depend on the inclination of the axis of rotation of a planet to the plane of its orbit around the sun. It depends on the presence or absence of a large moon (or moons) because, with a large and nearby moon, you get pronounced tides, while without a moon, or only very small moons, you only have the solar tide, which is likely to be unimpressive. The relative abundance of the chemical elements in the outer crust and in the atmosphere certainly also plays a role.

Let us, for a first test, take our two neighbors in the Solar System, Venus inside the Earth’s orbit and Mars outside it. When I started reading books on science, as a schoolboy, Venus, in most of them, was firmly declared to be a panthalassa, the technical term for a planet completely covered by water without any land showing. This, after various attempts to be “different,” has recently been revived by Whipple and Menzel as the most likely concept. Now such a shoreless ocean — I am avoiding all other consideration and am concentrating on just the one fact that it is an ocean — can harbor virtually everything in abundance. But with limitations; you can’t just mix the fauna of the equatorial Pacific Ocean of today with equatorial seas of the Jurassic and Cretaceous periods and obtain a believable or even possible picture.

You can have, if you want to, most of the arthropods, lobsters and sea spiders, trilobites and, if you insist, something like a seagoing centipede. But you must specify that there are shallow areas in this ocean if you want to have clams, for they don’t grow too far down. You can have jellyfish in fantastic numbers of species as well as individuals. You can have octopi and all sorts of fishes. But you can’t have a turtle, for example, because when, in Earth’s past, some fishes went up on land, they first produced what we now call amphibia — say, primitive salamanders — and the reptiles, the birds and the mammals came afterward. They all are creatures of the land, even though some reptiles, like the turtles and the sea snakes, and some mammals, like the whales and the seals, returned to the ocean at a later date.

And don’t make anything more intelligent than the most intelligent fish — I don’t know which fish that is or could be — for the open sea is a region of steady movement and no intelligence is needed for that. The exceptions to the statement that this is a region of movement are armored forms like clams, but a perfectly sessile creature which relies on its armor for individual protection and on numerous offspring for survival of the species also is not going to develop intelligence.

It doesn’t need any. So a shoreless Venusian ocean — I repeat I am concentrating on no other fact than that it is a shoreless ocean — might harbor a very varied life and some forms may be rather pretty. But I challenge anybody to think up an aquatic form of life, especially among the invertebrates, which would look radically different from what we have in our oceans.

The multitude of forms on our own planet is so overwhelming that one always gets the impression that anything that can survive with the shape it has is also in existence. One thing is absolutely needed in this shoreless ocean if it is to have any life at all. There must be plants, microscopic or otherwise, because animal life alone is an impossibility. You know the old tall tale about the man who made a living by having a mouse and cat farm. The cats, of course, ate the mice, and when the cats were big enough, he killed and skinned them, sold the pelts and fed the cat’s bodies to the mice.

Even if the mice were carnivorous, this just wouldn’t work. Somewhere at the beginning of such a cycle, there has to be the original food producer, the plant, which makes living (and edible, as a rule) tissue out of dissolved minerals, carbon dioxide and sunlight for energy. I might as well, at this point, present two strong hints at caution. If, in that sea, you have a tribe of Kraken, octopi a mile in circumference and the largest thing in the ocean, don’t make them smart. If they are the largest thing in the ocean, immune to all danger except an occasional outburst of the elements, such as a submarine volcano opening up, and, of course, old age, they don’t have to be intelligent. What has been said about oysters a while ago applies also to the invulnerable life-form. Likewise, don’t make something one millimeter in diameter into an intelligent life-form.

Some time ago, somebody wrote a story in which the main character, who was not a hero, caught what he thought to be a shiny wasp. It stung him so hard that he had to let go — and then noticed to his surprise that the wasp sting made his Geiger counter chatter wildly. The implication was, of course, that this was a tiny spaceship with atomic drive. Though I liked the story, I knew that this could never happen. In order to be intelligent enough to even discover atomic energy, a being has to have a rather large number of brain cells.

These brain cells must be nourished, which needs organs for eating and digesting food. The digestive tract must be protected by some covering and this package must be moved around in some manner so that it can find food.

It must also move around to avoid being eaten, at least until it has attained the intelligence that splits atoms and controls what they do after splitting. It has been said and bolstered with many pounds of statistics that, in a modern army, 98 men are needed to ehable two men to shoot at the enemy.

This relationship must apply also to the number of cells needed to support the brain cells that do the thinking. Since a cell, in order to function as a cell, must consist of a very large number of molecules and since the size of molecules is a given fact, there must be a minimum size for a functioning cell. Sprague de Camp, who was to my knowledge the first to present this chain of reasoning (in a two-part article in Astounding, May and June issues of 1939), came to the conclusion that an overall body weight of around 40 pounds would be needed if you want intelligence on the human level.

It is possible that a few facts permit a little more stretching, so that the minimum weight could be less. But the reasoning itself is valid and the reduction cannot be very much.

Whether the first interstellar hero has to establish relations with something weighing 45 or only 30 pounds does not make much of a difference. But I did not want to slip out of our solar system yet. Now if we look at Mars, we are helped no end by the fact that we know a great deal about it. Here is a small planet with very little water and a thin atmosphere consisting mostly of inert nitrogen. It is generally a cold planet, but during the summer the equatorial regions can attain temperatures between 60 and 70 degrees Fahrenheit at noon.

To make our problem still easier, we are virtually certain that we see plant life (this was written in those innocent days of 1956 when they still though Mars had visible signs of plant life, instead of the huge dust storms we now know are the case). The dark greenish patches which all bear nice classical names due to Signor Schiaparelli of half a century ago cannot just be mineral discolorations. When covered up by yellow dust from the deserts, they manage to break through again and just during the last close approach of Mars, in 1954, Dr. Slipher, working at Bloemfontein, South Africa, found a new one almost the size of Texas under about 15° northern Martian latitude and about 235° Martian longitude, which means about halfway between the northern end of Syrtis major and Trivium Charontis, two well-known Martian markings. There has been a lot of discussion recently in learned journals on whether any terrestrial plant could grow on Mars, and if so, which one.

Naturally any suggestion made by anybody was countered with heavy arguments by somebody else. But the fact remains that we see something growing on Mars which is, in our terminology, plant life.

If we do not understand their biochemistry under the conditions we are forced to assume from astronomical observations, this can only mean one of two things: Either we cannot observe all the conditions and something which we have missed, or are bound to miss with present instrumentation, is a perfectly fine explanation; or else we don’t know enough biochemistry and there is a way of living and growing under these conditions. The reasoning that forced us to say that there must be plant life in the Venusian oceans, if we want animal life of any kind, almost forces us to say that, since there are plants on Mars, there must be something that we would call animals. Some biologists with whom I discussed this stated with professional caution that this reasoning does not necessarily hold true.

I don’t agree. Speaking in the largest sense, the animals of Earth, from sow bugs to elephants, are parasitic on plants. Now life, at least on Earth, behaves in such a manner that if there is something to be parasitic on, something else will be happy to take over the role of the parasite.

Something feeding on these Martian plants must have the power of movement because it needs so much plant tissue for its own sustenance that the rate of the plant growth cannpt furnish the necessary amount. Hence it must be capable of locomotion.

Whether this supposed Martian plant-eater is built along the lines of a locust, or along the lines of a desert tortoise, or along those of a rabbit is something entirely different again. One can assume that it simply freezes into a deathlike state during the cold Martian rlight and remains in that state until thawed out by the Sun. That case, it could be insectlike in organization. One can assume with equal justification that the “animal,” at the first sign of cold in the evening, burrows into the ground for a few feet and goes to sleep normally in an environment where the temperature may be quite cold, but where there is very little deviation from whatever temperature it may have. In that case, it could be something comparable to a desert tortoise.

Or you can make the assumption that it has an internal mechanism like the birds and mammals of Earth, something producing heat. Then it does not have to dig itself in. All it needs is an effective heat insulator around its body, which might be hairlike, or featherlike,' or, if this sounds more “alien,” something like bark or sponge rubber. So far, I have mostly talked about extraterrestrial animal life in order to show some of the difficulties.

When it comes to an extraterrestrial intelligent lifeform, the difficulties rapidly increase in number and kind. It may come as a surprise, but the first tentative recipe for the construction of an intelligent extraterrestrial was written by the Dutch physicist, philosopher and astronomer Christian Huyghens. The title of the book is and it appeared posthumously, in 1692, at first in Latin. Nobody seems to know just when Huyghens wrote the major portion of the book. He said there that an extraterrestrial must have eyes and ears — that is, senses “and pleasure arising from his senses.” He must know the art of writing to remember things, arithmetic and geometry to understand relationships, hands to make things — and he must be upright. It does not become quite clear from Huyghens’ book why he must be upright. It sounds as if Huyghens made this condition to free the forelimbs from the task of locomotion so that there are “hands to make things.” The insistence struck me as amusing because Sprague de Camp, in the articles mentioned, also was insistent on that point, but more for mechanical reasons.

The brain must be protected against shock as much as possible and the more bone, cartilage and tissue there is between the feet, which take the shocks, and the brain, the better. All this is sound logic and it is obvious that the body of the extraterrestrial must be such that it functioned well as an animal body before it grew to be intelligent. Of course, one can postulate that accidental enviromental conditions of the past helped along. Around the turn of the century, a number of biologists and zoologists toyed with the idea that Man had evolved in what they called an asylum, an area accidentally free from large predatory animals and with a gentle climate. They obviously did not think much of the human body as a well-functioning animal. We now know that they were wrong and that the idea of the “asylum” is not needed.

But it may conceivably have happened somewhere else, for the Galaxy must be full of planets and possibilities. There is just one major difficulty in imagining a believable intelligent extraterrestrial — we have never seen one. What I mean by this remark is this: We know the organization of living animal tissue on Earth.

We know that the organization of the mammal is superior. True, it “wastes” food by making its own heat, but this fact makes it climatically independent. And though a reptile can do quite well in the proper climate, it is very limited. When the air grows too cold, it must be inactive, though it usually survives.

When the air grows too hot, it dies of heat stroke, for, lacking a temperatureregulating mechanism, it not only cannot keep warm, it also cannot keep cool. Now this vertebrate body, whether mammalian or reptilian, has two pairs of limbs and usually a tail. What we don’t know is Whether it has to be built that way. To use a classical example: we don’t know whether the centaur shape is possible or not. On Earth, it doesn’t exist; that much is certain. But is this due to an anatomical necessity for which we don’t know the reason or did it just happen that way here?

As for comparatively minor matters, we do know that they just happened. Genus Homo is tailless and almost hairless. But it doesn’t have to be hairless and tailless to invent writing, to build and ride cars and to engage in research, politics and crime. If we had fur and a tail, our fashions, habits and morals would be different, but if brain and senses and hands were unchanged, we’d still write books and symphonies, build houses, ships and airplanes — and try to build an extraterrestrial.

.you know how there are creatures that dwell in the most inaccessible, inhospitable places above, on and under the Earth and in her oceans? I am talking about life-forms you can find in any handbook of zoology, as opposed to those fearsome beings of the Cthulhu Cycle which which we are now so familiar.

Well, there are also creatures which exist in the most obscure and random corridors and corners of time, in lost and unthinkable abysses of space, and in certain other twilight places which are most easily explained by referring to them as junctions of forces neither temporal nor spacial, places which by all rights should only exist in the wildest imaginings of theoreticians and mathematicians.Suffice to say, then that there are extreme forms of life within and without this universe of ours. And I know it to be so for I have seen or learned of many such forms. For instance.intelligent energies in the heart of a giant alien sun who measure time in ratios of nuclear fission and space in unimaginable degrees of pressure!

There are wraithlike biological gasses which issue at the dark of their moon from the fissures of a fungoid world in Hydra, to dance away their brief lives until, exhausted, they die at dawn, scattering the sentient seeds of mushroom minds which will sprout and take root, and whose crevice-deep roots will in turn emit at the dark of the moon euphoric, spore-bearing mists of genesis. There is a dying purple sun on Andromeda's rim whose rays support life on all seven of its planets.

On the fourth planet there are exactly seventeen forms of life, or so it would appear. On closer inspection, however, a zoologist could tell you that these forms are all different phases of only one life-form! Consider the and cycles of Earth life and this might not seem too astonishing, until I tell you that of these seventeen phases two are as apparently inanimate mineral deposits, six are aquatic, two others amphibious, three land-dwelling cannibals, three more are aerial and the last is to all intents and purposes a plant while all of its preliminary stages (excluding the mineral phases) were animal. The starcraft gathered the fabric of time and space.

Chayn passed stars and groupings of stars, dense clusters of young stars and swirling clouds of dust and gas giving birth to new light in their depths. Black holes tunneled through the space-time structure into elsewhere, glowing ominously as matter spiraled down to annihilation. Chayn could perceive it all, but he focused his attention on the mind fields. Uncountable multitudes of worlds circles perhaps a third of the stars in his view. Most were lifeless, barren worlds of rock and snow, but even the tiny fraction that had given birth to life emanated a broad mind field that he could sense everywhere. There were worlds of microscopic life and paradises of forests and jungles teaming with dramas of life and death.

There were worlds ancient and wise in the ways of evolution, but what Chayn watched for were the sparks of intense awareness, life on levels near his own. Intelligence too far in advance of him were incomprehensible, aware of his passage, but apathetic. Most life forms on his own level were alien, different in inexplicable ways. He felt he could adapt to some of those strange and beautiful worlds if necessary, but he staved his hunger and waited for the worlds of man.

The Watcher told him that man had lived for eons, evolving to the greatness of the stargods, but that man in this galaxy had recently arrived in fleets of starships after sleeps of many millennia. The worlds of man were new here while Earth recycled its continents and evolved new species of life.Danger lay immediately ahead, a gulf of darkness between two arms of the galaxy. Chayn approached the starless void with caution. In that incredible abyss four hundred light years across, he could sense another kind of life -- the star travelers. He could sense such small concentrations of explorers only where they stood out like specks of brightness, even the blank minds of those who slept in the frozen oblivion of suspended animation. One of the star travelers in view piloted a starcraft similar to his own.

Two others were primitive vehicles of metal driven by fusion or antimatter-propulsion units to velocities below that of the speed of light. At first Chayn thought the pilot of the starcraft like his own would seek communication with him, but the entity was highly evolved and looked upon him as a curiosity. Chayn knew himself to be a primitive, more typical of the life forms frozen in their crude ships of metal.Chayn's fear intensified as he neared the abyss. Mindspiders lurked in the darkness, many species of them littering the void with invisible webs.

Some dangled thin and scraggly. Others spread magnificently, a light year in diameter. Even in that moment, he felt the shock, the utterly brilliant flare of terror of alien minds encountering the web in the far distance.

Particles rose to lethal intensities of radiation. Bodies died and the ship heated to incandescence. The mindspiders fed upon disembodied consciousness. Few of the primitives could perceive such danger lurking in the abyss.Ahead, he sensed an old, torn web. Even the mindspiders had their predators in their own realm.

This one was gone, the web deteriorating. : Well, hell, who doesn’t have some sort of bizarre senses? Especially since it gets very tricky if you count the whole electromagnetic spectrum as one – i.e., “ultravision” and “infravision” are both strict subsets of “vision”. As, for that matter, is sensing gamma rays – and other similar elisions. It’s not like anyone gets to claim the canonical radiation range for “sight”, now is it? But we’ve got people sensing everything from low infrared to high UV, with bioradio senses, with the ability to detect electromagnetic fields both static and changing, with the ability to feel the curvature of space-time (that would be those bionano vector-control effectors again), with echolocation and/or sonar, the ability to read plasmids by tasting them, and pretty much any other physical effect that you can measure somehow on the macroscale.

(The current eldrae alpha baseline clocks in at 24 recognized senses, by the way, counting the synthetic and transcendent ones, and that’s after considering smell and taste as one: photoception, audition, chemoception/olfaction, static mechanoception, dynamic mechanoception, thermoception, nociception, static electroception, dynamic electroception, proprioception, chronoception, farspeech, spatioception, secondary gestalt, secondary linear, mesh, metadata, worth, mnemonesis, nature, utility, entelechy, obligation, and autosentience.). A 'hive' intelligence would resemble an intelligent ant-hill, where each ant would be but a cell in the hill's 'body'. Individual ants may die, but the hill goes on. Examples include the 'Boaty Bits' from FARTHEST STAR by Jack Williamson and Frederik Pohl, the 'Godtalkers' from THE DRAGON NEVER SLEEPS by Glen Cook, the 'Tinker Composite' from THE MIND POOL by Charles Sheffield, the 'Mantis' from GREAT SKY RIVER by Gregory Benford, and the Martians from LAST AND FIRST MEN by Olaf Stapedon.

If the alien is composed of a hive of several species, it is some times called an 'anthology intelligence.' Go to The Tough Guide to the Known Galaxy and read the entry.

A good example of a hive intelligence was in Olaf Stapedon's classic Star Maker. The 'cells' composing an individual were free-flying birds linked telepathically. Birds might be born or die, but the flock-individual lived on. A more modest version were the 'Tines' in Vernor Vinge's A Fire Upon The Deep. One might even consider an anthill to be a hive organism, an individual who's cells are ants. A type of intelligent species - one of the most Really Alien of all - organized along lines rather like the social insects. In a Hive Entity, individuals members of the community count for nothing, and indeed most of them have no individual intelligence to speak of.

They are specialized for various functions (particularly warriors), and exist entirely to serve the Hive Entity as a whole. A Hive Entity's intelligence may reside in specialized 'brain' individuals, which have only vestigial legs and even digestive systems, and are themselves entirely dependent on various kinds of 'slave' individuals. Or the intelligence may somehow be spread out collectively though the whole Hive Entity, each individually-mindless inhabitant in effect contributing a few neurons to the whole. (Or some combination of these.) Some Hive Entities may not really be intelligent at all, but have evolved the ability to blow up other people's spacecraft the same way that some ants have evolved the ability to keep aphids as cattle. When encountered in the, Hive Entities are almost invariably hostile. They apparently have nothing to offer in trade, much less arts or ideas, and you can't even negotiate a peace treaty with them, because there isn't really anyone to negotiate with.

In they are at once mindlessly ruthless - attacking in endless waves like giant army ants, which they also tend to look like - and malevolently intelligent. Putting no value on their own automaton lives, they obviously have no concept of valuing anyone else's. In fact, Hive Entities are basically the ultimate totalitarians. It is no surprise that they appeared in written, so far as I know, around the mid 20th century, the same time that giant ants showed up in.

Hive Entities were, and are, Nazis, Stalinists, and ChiComs, magnified to the Nth degree and let loose to give better races a harsh lesson in the precious value of individualism. Which is really too bad.

Taken in themselves, Hive Entities are a fascinating concept, precisely because they really are Really Alien. Yet if in fact they are intelligent, they must have ideas of some sort, however hard for them to express in a way we can understand. If the intelligence is spread through the hive community, the time scale of its thinking might be drastically slower than our own, maybe taking weeks to form the equivalent of a sentence. This indeed could make them tricky to deal with at first, since on our time scale they would necessarily act on reflex. But if we, and similar species, really want to demonstrate individual intelligence, we might actually try figuring the Hive Entities out, and see if we and they might have something to contribute to each other, instead of fighting pretty mindless wars with them. Don't hold your breath, though. It hasn't happened in fifty years, so far as I know.

But maybe the Hive Entities' mental time scale is longer than that. I love social insects. Whether they’re ants, bees, termites, wasps,,, or, I find them fascinating to learn about. But if the sci-fi books I read as a kid had had their way, I should have run screaming from every ant colony I saw.

From the buggers in Ender’s Game to the Borg in Star Trek to the Vord in Codex Alera to ants and termites themselves from a morph’s-eye view in Animorphs, social insects, and the aliens or artificial intelligences that closely resemble them, are portrayed as “hive minds” with an emotional tone of existential terror. And I’m here to tell you that these portrayals are totally unfair. What they get right Here are some features that most portrayals of social insects and their analogues in sci-fi get right. Yes, social insect colonies have queens that are primarily responsible for reproduction.

Yes, social insects have very different sensory modalities from ours. We primarily use sight and sound to communicate and navigate the world, while social insects use taste and smell and vibration. Yes, social insects have specialized division of labor to particular tasks, and yes, they are willing to sacrifice themselves in droves to protect the colony. And sometimes, they will enslave social insects from other colonies or even species to serve their own ends (). Thus ends what sci-fi portrayals get right. What they get wrong: Queens Almost universally in sci-fi, when you kill the queen, the hive disintegrates into chaos.

You’ve cut off the head! The central intelligence of the hive is gone! They’re just mindless borg-units with no idea what to do!

Indeed, in some social insects, such as leafcutter ants, if you kill the queen, the whole colony will die – but probably not for the reasons you think. However, it’s more common for social insects to be able to carry on just fine regardless.

In most ants and bees, there are “backup” queens that are reared up by the workers in case the current queen should die. And in many social insects, a worker can step up and become a queen in her place. But here is the most important problem with the sci-fi trope of killing the queen to kill the hive. The queen is not the brain of the hive. She is the ovary. If you think of a social insect colony as a superorganism, which it’s useful to do in many cases, different groups of insects within the colony act like organs. One caste protects the colony from invaders, which is like an immune system.

One caste scouts for new places to forage, which is like a sensory system. Generally, science fiction has a good grip on this idea. Where sci-fi authors fail is that they think the queen is the brain of this superorganism. She is the reproductive system. The queen does not control what happens in the hive any more than your reproductive system controls what happens in your body. (Which is to say, she has some influence, but she is not the brains of the operation.) The reason why leafcutter ant colonies die when the queen dies is because the colony has been castrated, not beheaded. Most animals die when they are no longer able to reproduce, even if their brains are still perfectly functional.

For castrated colonies with no backup queen or gamergate and no hope of getting one, there is no point in carrying on. Their evolutionary line has ended. What they get wrong: Swarm intelligence Here is how social insect hive minds work in science fiction: the queen does the thinking, and the rest of the hive goes along with whatever she thinks. Now, I’ve already told you that the queen is not the brain of the hive.

So where is the brain? Well, that is exactly the point of swarm intelligence. The brain does not reside in one particular animal. It’s an emergent property of many animals working together. A colony is not like your body, where your brain sends an impulse to your mouth telling it to move, and it moves. It’s more like when two big groups of people are walking toward each other, and they spontaneously organize themselves into lanes so no one has a collision ().

There’s no leader telling them to do that, but they do it anyway. Much of the efficiency of social insect colonies comes from very simple behavioral rules (). Hymenopterans, the group of insects that includes ants, bees, and wasps, have a behavioral rule: work on a task until it is completed, and when it is done, switch to a different task. If you force solitary bees (yes, most bee species are solitary) to live together, they will automatically arrange themselves into castes, because when one bee sees another bee doing a task like building the nest, its behavioral rule tells it that the task is completed and it needs to switch to a different task, like looking for food. Individually, a social insect isn’t all that smart, whether it’s a queen, worker, soldier, or drone. But collectively, social insects can do incredibly smart things, like find the most efficient route from the colony to some food (), or choose the perfect spot to build their hive ().

What they get wrong: Individuality The existential terror of the hive mind in science fiction comes from the loss of the self. The idea is that in a social insect colony, there is no individual, but one whole, united to one purpose. No dissent, disagreement, or conflicting interests occur, just total lockstep. I totally get why that’s scary. The thing is, it’s just not true of real social insects. There is conflict within colonies all the time, up to and including civil war.

A common source of conflict within colonies is worker reproduction. Yes, in most social insects, workers can in fact reproduce, though usually they can only produce males. So why don’t they? Because it’s not in the interest of their fellow workers. Workers are more closely related to their siblings and half-siblings produced by the queen than they are to their nephews, so they pass on more of their genes if they spend resources on raising the queen’s eggs. So, if a worker catches its fellow laying an egg, it will eat the egg. Not exactly “all for one and one for all,” is it?

Worker insects may also fight in wars of succession. If there is more than one queen in a species where queens do not tolerate each other (yes, there are species where multiple queens get along together just fine), such as monogynous fire ants, the workers will ally themselves with one queen or another and engage in very deadly civil war.

Finally, in some species, the queen needs to bully the workers into doing their jobs, and the dominant workers need to bully subordinate workers into doing their jobs (). Yes, sometimes workers try to laze around and mooch.

Surprisingly human Here’s what I find weird about depictions of social insects in science fiction. They are portrayed as utterly alien, Other, and horrifying. Yet humans and social insects are very, very similar. The famous sociobiologists E.O. Wilson and Bernard Crespi have both described humans as chimpanzees that took on the lifestyle of ants.

I think what fascinates people, including me, about ants, bees, and their ilk is that you watch, say, a hundred ants working together to tear up a leaf into tiny bits and carry it back to their colony, or a hundred bees all appearing out of seemingly nowhere to sacrifice themselves en masse to stop a bear from eating their hive, and it looks like magic. It really does look like some kind of overmind is controlling their collective actions.

But imagine you’re an alien who comes to Earth, and you know nothing about humans or the way we communicate. Wouldn’t we look exactly the same to them as ants and bees look to us? Wouldn’t they look at us sacrificing our lives by the thousands in wars, or working together to build cities from nothing, and think, Wow, how do they coordinate themselves in such huge numbers, why do they give up their lives to defend their borderlines, I guess there must be some kind of mega-brain they all share that tells them what to do, and they just march in lockstep and do it. If there’s anything I’ve learned from the study of both social insects and humans, it’s that any system that looks monolithic and simple from a distance is in fact fractured, messy, and complicated when you look at it up close.

Social insects aren’t scary mindless robot-aliens. They’re a lot like you and me. As much as I was terrified as a kid by the Animorphs book where an ant morphs into Cassie and screams in pure existential horror at its sudden individuality, I actually think an ant would adjust very easily to being a human, and that a human would adjust very easily to being an ant — much more easily, in fact, than humans adjusted to morphing, say, sharks, in the very same book series. Aliens with wings is a popular trope. The dream of flying like a bird is probably been around since the dawn of recorded history. But you can forget about. You see, wings need to flap with enough power to lift the person.

The power comes from muscles, lots of muscles. So much muscle in fact that in birds they need a special bone for the wing muscles to attach to.

This is called the. The muscles are what we call the meat of a chicken or turkey breast, and the keel is the breastbone. I trust you can spy the problem. A humanoid with wings is going to have a deformed chest that looks like the prow of a huge boat.

And female humanoids with wings will not have mammary glands. Not on their chest at any rate. That segment of the science fiction audience with the personalties of adolescent boys will be angry at the lack of. Once again the fans will be outraged at scientific accuracy. And they will vote with their wallet. As you can see while it is not actually impossible to have humanoid winged creatures, they are going to be more towards the 'noid' and less towards the 'human' part of the spectrum.

Which will put them right in the, inspiring revulsion instead of attraction. They ain't gonna look like angels. A more minor problem is the fact that on a bird, the wings are basically its arms. On humans, arms are attached to the shoulder blades.

Which means a winged humanoid with both arms and wings is going to need four shoulder blades, not the customary two. Which probably means the wings will be attached to the small of the back, not the shoulders. Also the neck should be long and articulated so when flying (and basically in a prone position) it can bend the head so it can see where it is going, instead of being forced to look at the ground. • Four shoulder blades Concept and art by • Long articulated neck Concept and art. Maybe his irritation with the pilot spoke for Webner: 'How often must I explain there is no such risk, yet? Instead, here's a chance to learn.

What happens next could give us invaluable clues to understanding the whole ethos. To Turekian: 'Forget about that alleged metal. Could be protective collars, I suppose.

But take the supercharger off your imagination.' The other man froze where he stood.

Yukiko seized his arm. He stared beyond her. 'What's wrong?' He shook himself.

' Supercharger,' he mumbled. 'By God, yes.' Abruptly, in a bellow: 'We're leaving! They are the dwellers, and they've gathered the whole countryside against us!' When you know what to expect, a little, you can lay plans. We next sought the folk of Ythri, as the planet is called by its most advanced culture, a thousand kilometers from the triumph which surely prevailed in the mountains.

Approached with patience, caution, and symbolisms appropriate to their psyches, they welcomed us rapturously. Before we left, they'd thought of sufficient inducements to trade that I'm sure they'll have spacecraft of their own in a few generations.

Still, they are as fundamentally territorial as man is fundamentally sexual, and we'd better bear that in mind. The reason lies in their evolution. It does for every drive in every animal everywhere.

The Ythrian is carnivorous, aside from various sweet fruits. Carnivores require larger regions per individual than herbivores or omnivores do, in spite of the fact that meat has more calories per kilo than most vegetable matter. Consider how each antelope needs a certain amount of space, and how many antelope are needed to maintain a pride of lions. Xenologists have written thousands of papers on the correlations between diet and genotypical personality in sophonts. I have my doubts about the value of those papers. At least, they missed the possibility of a race like the Ythrians, whose extreme territoriality and individualism—with the consequences to governments, mores, arts, faiths, and souls—come from the extreme appetite of the body. They mass as high as thirty kilos; yet they can lift an equal weight into the air or, unhampered, fly like demons.

Hence they maintain civilization without the need to crowd together in cities. Their townspeople are mostly wing-clipped criminals and slaves. Today their wiser heads hope robots will end the need for that. The original talons, modified for manipulating. Those claws on the wings, a juvenile feature which persisted and developed, just as man's large head and sparse hair derive from the juvenile or fetal ape. The forepart of the wing skeleton consists of humerus, radius, and ulnar, much as in true birds. These lock together in flight.

Aground, when the wing is folded downward, they produce a 'knee' joint. Bones grow from their base to make the claw-foot. Three fused digits, immensely lengthened, sweep backward to be the alatan which braces the rest of that tremendous wing and can, when desired, give additional support on the surface. To rise, the Ythrians usually do a handstand during the initial upstroke. It takes less than a second. Oh, yes, they are slow and awkward afoot.

They manage, though. Big and beweaponed, instantly ready to mount the wind, they need fear no beast of prey. You ask where the power comes from to swing this hugeness through the sky. The oxidation of food, what else? Hence the demand of each household for a great hunting or ranching demesne. The limiting factor is the oxygen supply.

A molecule in the blood can carry more than hemoglobin does, but the gas must be furnished. Turekian first realized how that happens. The Ythrian has lungs, a passive system resembling ours. In addition he has his, evolved from the gills of an amphibianlike ancestor. Worked in bellows fashion by the flight muscles, connecting directly with the bloodstream, those air-intake organs let him burn his fuel as fast as necessary. A shape blotted out the sun. They bounded to their feet.

That which was descending passed the disc, and light blazed off the gold-bronze pinions of a six-meter wingspan. Air whistled and thundered.

Fraina cried out. Mikkal poised his javelin.

Ivar shouted. He's Ythrian!' 'O-o-oh, ye-e-es,' Mikkal said softly.

He lowered the spear though he kept it ready. Fraina gripped Ivar's arm and leaned hard against him.

The being landed. Ivar had met Ythrians before, at the University and elsewhere. But his astonishment at this arrival was such that he gaped as if he were seeing one for the first time. Grounded, the newcomer used those tremendous wings, folded downward, for legs, claws at the bend of them spreading out to serve as feet, the long rear-directed bones lending extra support when at rest.

That brought his height to some 135 centimeters, mid-breast on Ivar, farther up on the tinerans; for his mass was a good 25 kilos. Beneath a prowlike keelbone were lean yellow-skinned arms whose hands, evolved from talons, each bore three sharp-clawed fingers flanked by two thumbs, and a dewclaw on the inner wrist. Above were a strong neck and a large head proudly held.

The skull bulged backward to contain the brain, for there was scant brow, the face curving down in a ridged muzzle to a mouth whose sensitive lips contrasted curiously with the carnivore fangs behind. A stiff feather-crest rose over head and neck, white edged with black like the fan-shaped tail. Otherwise, apart from feet, arms, and huge eyes which burned gold and never seemed to waver or blink, the body was covered with plumage of lustrous brown. He wore an apron whose pockets, loops, and straps supported what little equipment he needed. Knife, canteen, and pistol were the only conspicuous items. He could live off the country better than any human The Anglic which replied was sufficiently fluent that one couldn't be sure how much of the humming accent and sibilant overtones were due to Ythrian vocal organs, how much simply to this being an offplanet dialect the speaker had learned. 'Thanks, greetings, and fair winds wished for you.

I hight Erannath, of the Stormgate choth upon Avalon. Let me quench thirst and we can talk if you desire.' As awkward on the ground as he was graceful aloft, he stumped to the pool. When he bent over to drink, Ivar glimpsed the gill-like antlibranchs, three on either side of his body. They were closed now, but in flight the muscles would work them like bellows, forcing extra oxygen into the bloodstream to power the lifting of the great weight. That meant high fuel consumption too, he remembered. No wonder Erannath traveled alone, if he had no vehicle.

This land couldn't support two of him inside a practical radius of operations Mikkal settled himself back in the shade where he had been. 'Might I ask what brings you, stranger?' 'Circumstances,' Erannath replied. His race tended to be curt. A large part of their own communication lay in nuances indicated by the play of marvelously controllable quills Expressions they could not read rippled across the feathers 'A sophont,' Mikkal said redundantly.

He proceeded: 'More bright and tough than most. Maybe more than us.

Could be we're stronger, we humans, simply because we outnumber them, and that simply because of having gotten the jump on them in space travel and, hm, needing less room per person to live in.' 'No,' Ivar told her. 'They're feathered, yes, warm-blooded, two sexes. However, you noticed he doesn't have a beak, and females give live birth.

No lactation—no milk, I mean; the lips're for getting the blood out of prey.' A canned lecture was barely under way. A human xenologist stood in the screen and intoned: 'Warm-blooded, feathered, and flying, the Ythrians are not birds; they bring their young forth viviparously after a gestation of four and a half months; they do not have beaks, but lips and teeth.

Nor are they mammals; they grow no hair and secrete no milk; those lips have developed for parents to feed infants by regurgitation. And while the antlibranchs might suggest fish gills, they are not meant for water but for—' He reactivated the screen. It showed an Ythrian walking on the feet that grew from his wings: a comparatively slow, jerky gait, no good for real distances. The being stopped, lowered hands to ground, and stood on them. He lifted his wings, and suddenly he was splendid.

Beneath, on either side, were slits in column. As the wings rose, the feathery operculum-like flaps which protected them were drawn back.

The slits widened until, at full extension, they gaped like purple mouths. The view became a closeup. Thin-skinned tissues, intricately wrinkled, lay behind a curtain of cilia which must be for screening out dust.

When the wings lowered, the slits were forced shut again, bellows fashion. The lecturer's voice said: 'This is what allows so heavy a body, under Terra-type weight and gas density, to fly. Ythrians attain more than twice the mass of the largest possible airborne creature on similar planets elsewhere. The antlibranchs, pumped by the wing-strokes, take in oxygen under pressure to feed it directly to the bloodstream. Thus they supplement lungs which themselves more or less resemble those of ordinary land animals. The Ythrian acquires the power needed to get aloft and, indeed, fly with rapidity and grace.'

The view drew back. The creature in the holograph flapped strongly and rocketed upward.

'Of course,' the dry voice said, 'this energy must come from a correspondingly accelerated metabolism. Unless prevented from flying, the Ythrian is a voracious eater. Aside from certain sweet fruits, he is strictly carnivorous. His appetite has doubtless reinforced the usual carnivore tendency to live in small, well-separated groups, each occupying a wide territory which instinct makes it defend against all intruders. 'In fact, the Ythrian can best be understood in terms of what we know or conjecture about the evolution of his race.' 'We believe that homeothermic—roughly speaking, warm-blooded—life on Ythri did not come from a reptilian or reptiloid form, but directly from an amphibian, conceivably even from something corresponding to a lungfish.

At any rate, it retained a kind of gill. Those species which were most successful on land eventually lost this feature.

More primitive animals kept it. Among these was that small, probably swamp-dwelling thing which became the ancestor of the sophont. Taking to the treetops, it may have developed a membrane on which to glide from bough to bough. This finally turned into a wing.

Meanwhile the gills were modified for aerial use, into superchargers.' 'As usual,' Wa Chaou observed. 'The failures at one stage beget the successes of the next.' 'Of course, the Ythrian can soar and even hover,' the speaker said, 'but it is the tremendous wing area which makes this possible, and the antlibranchs are what make it possible to operate those wings. 'Otherwise the pre-Ythrian must have appeared fairly similar to Terran birds.'

Pictures of various hypothetical extinct creatures went. 'It developed an analogous water-hoarding system—no separate urination—which saved weight as well as compensating for evaporative losses from the antlibranchs. It likewise developed light bones, though these are more intricate than avian bones, built of a marvelously strong two-phase material whose organic component is not collagen but a substance carrying out the functions of Terra-mammalian marrow. The animal did not, however, further ease its burdens by trading teeth for a beak. Many Ythrian ornithoids have done so, for example the uhoth, hawklike in appearance, doglike in service.

But the pre-sophont remained an unspecialized dweller in wet jungles. 'The fact that the young were born tiny and helpless—since the female could not fly long distances while carrying a heavy fetus—is probably responsible for the retention and elaboration of the digits on the wings. The cub could cling to either parent in turn while these cruised after food; before it was able to fly, it could save itself from enemies by clambering up a tree.

Meanwhile the feet acquired more and more ability to seize prey and manipulate objects. 'Incidentally, the short gestation period does not mean that the Ythrian is born with a poorly developed nervous system. The rapid metabolism of flight affects the rate of fetal cell division. This process concentrates on laying down a body pattern rather than on increasing the size. Nevertheless, an infant Ythrian needs more care, and more food, than an infant human. The parents must cooperate in providing this as well as in carrying their young about.

Here we may have the root cause of the sexual equality or near equality found in all Ythrian cultures. 'Likewise, a rapid succession of infants would be impossible to keep alive under primitive conditions. This may be a reason why the female only ovulates at intervals of a year—Ythri's is about half of Terra's—and not for about two years after giving birth. Sexuality does not come overtly into play except at these times. Then it is almost uncontrollably strong in male and female alike. This may well have given the territorial instinct a cultural reinforcement after intelligence evolved. Parents wish to keep their nubile daughters isolated from chance-met males while in heat.

Furthermore, husband and wife do not wish to waste a rich, rare experience on any outsider. 'The sexual cycle is not totally rigid. In particular, grief often brings on estrus.

Doubtless this was originally a provision of nature for rapid replacement of losses. It seems to have brought about a partial fusion of (sex+death motif. In Freudian psychology these are opposed, apparently in Ythrian psychology they are fused) in the Ythrian psyche which makes much of the race's art, and doubtless thought, incomprehensible to man. An occasional female can ovulate at will, though this is considered an abnormality; in olden days she would be killed, now she is generally shunned, out of dread of her power. A favorite villain in Ythrian story is the male who, by hypnosis or otherwise, can induce the state. Of course, the most important manifestation of a degree of flexibility is the fact that Ythrians have successfully adapted their reproductive pattern, like everything else, to a variety of colonized planets.'

'But to return to evolution,' the lecturer was saying. 'It seems that a major part of Ythri underwent something like the great Pliocene drought in Terra's Africa. The ornithoids were forced out of dwindling forests onto growing savannahs. There they evolved from carrion eaters to big-game hunters in a manner analogous to pre-man. The original feet became hands, which eventually started making tools.

To support the body and provide locomotion on the ground, the original elbow claws turned into feet, the wings that bore them became convertible to legs of a sort. 'Still, the intelligent Ythrian remained a pure carnivore, and one which was awkward on land. Typically, primitive hunters struck from above, with spears, arrows, axes. Thus only a few were needed to bring down the largest beasts. There was no necessity to cooperate in digging pits for elephants or standing shoulder to shoulder against a charging lion.

Society remained divided into families or clans, which seldom fought wars but which, on the other hand, did not have much contact of any sort. 'The revolution which ended the Stone Age did not involve agriculture from the beginning, as in the case of man. It came from the systematic herding, at last the domestication, of big ground animals like the maukh, smaller ones like the long-haired mayaw. This stimulated the invention of skids, wheels, and the like, enabling the Ythrian to get about more readily on the surface. Agriculture was invented as an ancillary to ranching, an efficient means of providing fodder. The food surplus allowed leisure for travel, trade, and widespread cultural intercourse. Hence larger, complex social units arose.

'They cannot be called civilizations in a strict sense, because Ythri has never known true cities. The mobility of being winged left no necessity for crowding together in order to maintain close relationships. Granted, sedentary centers did appear—for mining, metallurgy, and other industry; for trade and religion; for defense in case the group was defeated by another in aerial battle. But these have always been small and their populations mostly floating. Apart from their barons and garrisons, their permanent inhabitants were formerly, for the main part, wing-clipped slaves—today, automated machines. Clipping was an easy method of making a person controllable; yet since the feathers could grow back, the common practice of promising manumission after a certain period of diligent service tended to make prisoners docile. Hence slavery became so basic to pre-industrial Ythrian society that to this day it has not entirely disappeared.'

Between these investigations, he caught momentary glimpses of the city, and realized how difficult—and dangerous—it would be for him to travel around in it. Streets were practically non-existent, and there seemed to be no surface transport. This was the home of creatures who could fly, and who had no fear of gravity. It was nothing to come without warning upon a vertiginous drop of several hundred metres, or to find that the only entrance into a room was an opening high up in the wall. In a hundred ways, Jan began to realize that the psychology of a race with wings must be fundamentally different from that of earthbound creatures. It was strange to see the Overlords flying like great birds among the towers of their city, their pinions moving with slow, powerful beats. And there was a scientific problem here.

This was a large planet—larger than Earth. Yet its gravity was low, and Jan wondered why it had so dense an atmosphere.

He questioned Vindarten on this, and discovered, as he had half expected, that this was not the original planet of the Overlords. They had evolved on a much smaller world and then conquered this one, changing not only its atmosphere but even its gravity.

The architecture of the Overlords was bleakly functional; Jan saw no ornaments, nothing that did not serve a purpose, even though that purpose was often beyond his understanding. If a man from mediaval times could have seen this red-lit city, and the beings moving through it, he would certainly have believed himself in Hell. Even Jan, for all his curiosity and scientific detachment, sometimes found himself on the verge of unreasoning terror. The absence of a single familiar reference point can be utterly unnerving even to the coolest and clearest minds. The city marched up out of the crimson haze, ever more awful, the bulk of it swelling to blot out half the red sky with gleaming black metal, the titanic machines that crowned it frowning down with the threat of unknown death. A palpable atmosphere of dread and horror hung over that unearthly metropolis, a sense of evil power and hostile strength, of ancient wisdom and monstrous science, for it had endured since the Earth was new. The four ragged creatures on the raft gazed on those marching walls with a hopeless horror.

Their minds sank prostrate with realization that unless their puny efforts could free the girl imprisoned there, the makers of this pile of black metal had also shaped the doom of mankind. The city seemed dead at first, a somber necropolis, too old for any life. But presently they saw movement along the walls.

A black spider-ship spread titanic vanes, and rose silently from a high platform to vanish in the red sky eastward. 'We must cover ourselves,' said Jay Kalam.

'They might be watching.' He had them screen the raft with broken branches, to look like driftwood. And the river carried them on toward the mighty wall. They were gazing upward in awe­struck silence when Hal Samdu cried: 'See them moving! Above the wall!' And the others could presently distinguish the creatures that moved—still tiny with many miles of distance—the ancient masters of this aged planet! John Star had glimpsed one of the Medusas on Mars, that thing in the gondola swung from the black flier, whose weapon had struck him down.

A swollen, greenish surface, wetly heaving; a huge, ovoid eye, luminous and purple. But these were the first he had fully seen. They drifted above the wall like little green balloons. Their eyes were tiny dark points in their bulging sides—each had four eyes, spaced at equal distances about its circumference. From the lower, circular edge, like the ropes that would have suspended the car of a balloon, hung a fringe of black and whiplike tentacles.

John Star could see the superficial likeness, the dome shape, the fringing tentacles, that had earned them the name Medusae. In the distance they did not look impressive. There was about them a certain grotesq«eness, a slow awkwardness.

They didn't look intelligent. Yet in the way they moved, floating apparently at will above the black wall, was a power and mystery that made for respect. And in the knowledge that they were the builders of this black metropolis was room for awe and terror. Scrambling over the immense bearing of the shaft, they found a little circular hole in the roof of the tank—it must have been left for attention to the bearings.

They climbed through it, Giles Habibula sticking until the others pulled him out, and so at last, on top of the reservoir, they were fairly within the city. They stood on the lower edge of a conical black metal roof, a dizzy drop of two thousand feet below them, and the slope too steep for comfort.

Standing there on that perilous brink, John Star felt a staggering impact of nightmare strangeness and bewildering confusion. Buildings, towers, stacks, tanks, machines, all loomed up about him, a black fantastic forest against the lurid sky, appallingly colossal. The tallest structures reached, he soberly estimated, two miles high. If this black metropolis of the monstrous Medusae had order or plan, he did not grasp it.

The black wall had seemed to enclose a regular polygon. But within all was strange, astounding, incomprehensible, to the point of stunning dismay. There were no streets, but merely yawning cavernous abysms between mountainous black structures. The Medusae had no need of streets.

They didn't walk, they floated! Doors opened upon sheer space, at any level from the surface to ten thousand feet. The stupendous ebon buildings had no regular height or plan, some were square, some cylindrical or domed, some terraced, some—like the reservoir upon which they stood—sheerly vertical. All among them were bewildering machines of unguessable function—save that a few were apparently aerial or interstellar fliers, moored on landing stages—but all black, ugly, colossal; dread instrumentalities of a science older than the life of Earth. Artwork by Jack Gaughan Aliens with wheels are a. There are problems with, and even more problems finding a plausible sequence where such a thing. In the real world the closest thing to an animal with wheels is the spinning of certain microscopic bacteria.

Wheeled aliens make an appearance in the satirical 'Retief' story, the g'Kek of (looking like 'a squid in a wheelchair' that suffer from arthritic axles when elderly), and in the Polarians of the novels (technically the Polarians do not use wheels, they roll around on large spheres). A milder version is rolling aliens. They are generally shaped like a sphere or a disc harrow, the entire alien rolls instead of just part of the alien. There is a spherical alien in and, a cylindrical alien in, a disembodied wheel in, and disc harrow aliens called the Slash of the novels •. The boy slowed.

An alien was squatting in the path. They drew up before the strange creature. It was a teardrop-shaped thing with a massive spherical wheel on the bottom and a limber tentacle or trunk at the top.

When that tentacle reached straight up, it would be as high as Flint, and the body's mass was similar to his. But the Polarian had no eyes, ears, nose, or other appendages. The Shaman claimed they were similar to human beings because they liked similar gravity, breathed the same air—though they had no lungs—and had a similar body chemistry. Their brains were as massive and versatile as man's, and they were normally inoffensive.

But they looked quite different, and such details as how they ate, reproduced, and eliminated were mysteries. But Flint had promised himself to treat the next alien he met with special courtesy. He and the boy halted politely.

'Greetings, explorer,' Flint said. The creature's body glowed with simulated pleasure. It put its stalk down to the ground.

In this position it looked more than ever like a dinosaur dropping. Flint stifled a laugh.

A little ball in the tip of the trunk spun rapidly. 'Greetings, native,' the ground said. Flint was not surprised. He had been familiar with the mechanism from infancy. The little ball vibrated against the ground—or any available surface—to produce intelligible sounds. As the Polarian had no mouth, it could not talk as humans did.

Now he had done it! He had never suspected the creature would accept! Well, it couldn't be helped. 'It is an emergency.

We shall be hurrying.' 'I shall not impede you,' the Polarian replied. But Flint smiled graciously.

He gestured to the boy. 'Show the way.' The runner was off, sensing a race.

This was firm, level ground, excellent for making time. Flint followed, stretching his legs. But Tsopi followed right along, rolling smoothly on her ball-wheel. She was at no disadvantage. Polarians could move rapidly and effortlessly when the terrain was right; their wheel was efficient. Flint had not before appreciated how efficient. On occasion he had wondered how the aliens kept themselves upright.

The Shaman had remarked that a man on a unicycle performed the same feat. But there were no unicycles on Outworld. How did no-handed creatures manage to build such edifices?

Again his memory provided the answer: Polarians were adept at circular manipulation of objects and concepts. They did not carry building blocks into place, they rolled building spheres into place. Where men laid bricks, Polarians rolled stones. Where men hammered nails, Polarians squeezed glue.

The end result was rather similar, as though civilization shaped itself into certain configurations regardless of the sapient species invoking it. Here there were no square skyscrapers, but domed dunes serving the same purpose. They passed down a smooth ramp, where on Earth there would have been stairs. Of course; ramps were better for wheels, stairs for legs. Ramps were everywhere, contributing to the fluidity of the architectural design. They had to roll single file, for efficient progress through the throng.

Tsopi's trail just ahead of him was sweet; she had a tantalizingly feminine taste. Flint concentrated, and it came: Polarians laid down taste trails with their wheels, much as humans laid down scent. No, more than that: These were actual, conscious signatures of passage, like the trails of Earthly snails. He remembered the first snail he had seen, beside the huge water of the ocean inlet, under the odd blue sky of Earth. Today he didn't even notice the color of the sky of a given planet; sky was sky color, right for its world.

But this taste; every Polarian was really a super-bloodhound, sniffing out every other, all the time. It was the natural way. In fact, it was already difficult to imagine how it could be otherwise. The monster charged, when Herald was off-balanced from his effort. And suddenly he realized another point of affinity: the monster was like a Slash, his own kind!

A Slash was a tubular creature with disks around its girth that it used for slicing out pathways, cutting up food, and dismembering enemies. It also had laser lenses for longer-range action. In his natural body, Herald could have met this creature on even terms, perhaps more than even terms.

A Slash was smaller, but the lasers could score with devastating effect before the disks struck. But this Solarian host was a poor excuse for a combat creature.

A is a 'flexible, mobile, elongated organ present in some species of animals, most of them invertebrates' (technical term ). Since they are uncommon in familiar earthly animals, they became a popular characteristic on unearthly science fiction aliens. This old trope dates back to prehistory, when the first man was freaked out when they discovered the octopus. In science fiction it dates back to at least 1898 with H. Well's War of the Worlds. It lingers on in popular media.

TV Tropes notes how be-tentacled creatures commonly use their tentacles in unique ways for (such as the old, that never gets old). As are tropes about the unexpected vulnerabilities of tentacles, such as the ' trope.

According to Dr. Moravec of the Carnegie-Mellon University, most land animals on Terra are '.' Once upon a time animals were shaped like sticks (worms), and couldn't manipulate or even locomote very well.

Then the sticks grew smaller sticks (arms and legs) and locomotion was much improved, and manipulation a little. Then the smaller sticks grew yet smaller sticks (fingers), and hands were invented, and manipulation got better. Generalize the concept.

I visualize a robot that looks like a tree, with a big stem, repeatedly branching into thinner, shorter and more numerous twigs, finally ending up in jillions of near-microscopic cilia. Each intermediate branch would have three or four degrees of freedom, an azimuth-elevation mount at its base, and an axial rotation joint at the top, where it connects to the next level of smaller twigs, and possibly also a length altering telescoping joint. To a large extent fewer degrees of freedom per level can be traded off for more levels. Each branch would also incorporate force sensing. Though each branch would be a rigid 'mechanical' object, the overall structure would have an 'organic' flexibility because of the great multitude of degrees of freedom.

From So species that use tentacles figured out how to turn an arm or leg into a manipulative organ without needing to grow fingers. Mechanically a tentacle is a ', consisting mainly of muscles with no skeletal support (an arm with no bones). It relies on the fact that water is effectively incompressible at physiological pressures, and the fact that muscles are mostly composed of water ( i.e., it is ). If the structure used pockets of water in separate compartments instead of watery muscles it would be a, but I digress. Common examples of muscular hydrostats include octopus tentacles, elephant trunks, the entire body of a worm, and the human tongue. STRUCTURE Tentacles are mostly solid muscle.

Just like in animals with skeletons or exoskeletons, tentacle muscles can only provide force by contracting, expanding doesn't do diddly squat. So just like in conventional animal limbs all tentacle muscles are arranged in. If one muscle pulls to the left it is paired with an antagonist muscle that pulls to the right.

As one muscle in the pair contracts the other relaxes. The muscle fibers are oriented in three different directions. MUSCLE ARRANGEMENT WITHIN THE OCTOPUS ARM VIOLET: Transverse Muscles BLUE: Circular Muscles GREEN: Longitudinal Muscle RED: External Helical Muscles ORANGE: Medial Helical Muscles YELLOW: Internal Helical Muscles WHITE: Axial Nerve Cord Dome structure at lower right is a cross section of a adapted from The closer the longitudinal muscles are located to the tentacle skin, the more elaborate bending movements are possible. Octopus arms, elephant snouts, and other manipulators all have this arrangement. You only see centrally located longitudinal muscles in limbs that just protrude in and out, like snake and anteater tongues. Muscles perpendicular to the long axis can be in a circular, radial, or transverse pattern. Radial and transverse muscles are anchored to the external connective tissue by threads called 'trabeculae' which penetrate the longitudinal and helical muscles that are in the way.

Transverse muscles are in sheets that alternate between horizontal and vertical (the 'down' direction is towards the side of the tentacle with suckers, technical term is 'oral side'). Perpendicular Muscles Type Orientation Examples Circular Rings around long axis squid tentacle mammal tongue Radial Radiating from center in a disk shape chambered nautilus tentacle elephant trunk Transverse Alternating between horizontal and vertical octopus tentacle human tongue Helical or oblique fibers wrap around the long axis like stripes. They are usually in two or more layers of opposite (left hand/right hand). The external and medial helixes are at an angle of 50 to 60° to the long axis, internal are at 40 to 50°.

The role of the internal helical muscles is unclear. MOTIONS Like all, the operating principle is the incompressibility of water, that is, if you push water into one end of a tube water will come spraying out of the other end. The important point is 'incompressible' means the volume of water always stays the same. If you reduce a volume of water's dimension in one direction it will have to expand in at least one other dimension.

So, for instance, if the muscles squeeze the tentacle to reduce its diameter (height and width dimensions), the tentacle will elongate along the long axis (length dimension). Because the volume of tentacle has to always stay the same. Elongation and Shortening This is when the tentacle grows or shrinks along the long axis. Like when you stick out your tongue. When the perpendicular (or helical) muscles contract (decreasing the tentacle's diameter) it elongates along the long axis (increasing the length). When the longitudinal muscles contract the tentacle shortens along the long axis (shortening the length) while simultaneously expanding hight and width (increasing the diameter).

So in this case the perpendicular muscles are operating to the longitudinal muscles. Some frogs can elongate their tongues up to 180% of its resting length. Due to hydraulics, the more the tongue is capable of elongating, the less force it can hit an object with.

Bending Bending the tentacle is done by using the longitudinal muscle to reduce the length of the tentacle while other muscles act to prevent the length reduction on one side of the tentacle. This causes a bend on the opposite side of the tentacle.

Octopi apparently contract all of the longitudinal muscles while strategically using the perpendicular muscles to maintain a constant diameter at specific points. Some tentacle robot limb designs have no perpendicular muscles. Instead they expand (using inflatable tubes instead of muscles) only some of the longitudinal muscle.

The robot tentacle bends on the side of the uninflated tubes. Torsion This is twisting the tentacle on the long axis, like it was a drill bit. It is done by contracting one of the two sets of hexlical muscles.

From Stiffening It is possible to make the tentacle rigid. The details are elusive but I would presume it can be done by contracting all the muscles at once. MOVEMENT CATEGORIES Reaches Moving the tentacle to increase the distance between tentacle tip and tentacle base.

The two basic types of reaches are: Uncurling Reach: where the tentacle starts out rolled up in a spiral and rolls out. Elongating Reach: where the arm starts out straight and grows longer. Pulls Moving the tentacle to decrease the distance between tentacle tip and tentacle base. The three basic types of pulls are: Continuum Curling Pull: where the arm rolls into a spiral. Straight-arm Shortening: where the arm is straight and grows shorter. Bending Pull: where the arm creates an elbow like bending point.

Searches/Gropes/Explores These are behaviors that are a lateral combination of sharp bends, sweeps, wraps, lifts, torsional rotations, drop, etc. Penn State Research Team Develops OctArm Soft Robot Manipulator Recent interest in expanding the capabilities of robot manipulators has led to significant research in continuum manipulators. The idea behind these robots is to replace the serial chain of rigid links in conventional manipulators with smooth, continuous, and flexible links. Unlike traditional rigid-linked robots, continuum robot manipulators can conform to their surroundings, navigate through unstructured environments, and grasp objects using whole arm manipulation. Soft continuum manipulators can be designed with a large number of actuators to provide hyper-redundant operation that enables dexterous movement and manipulation with robust performance. This improved functionality leads to many applications in industrial, space, and defense robotics.

Previous continuum robots used cable-tendon and pressurized tube actuators with limited performance. Cable-tendons must be tensioned or the cables become snarled or fall off drive pulleys, limiting the robot speed. Pneumatic bellows have low shear stiffness, limiting load capacity. Thus, there exists a need for a highly dexterous, fast, and strong soft robot manipulators.

Christopher Rahn, Professor of Mechanical Engineering at Penn State along with his students Dustin Dienno and Mike Pritts, and assisted by Dr. Michael Grissom developed the OctArm manipulator using air muscle actuators. These actuators are constructed by covering latex tubing with a double helical weave, plastic mesh sheath to provide the large strength to weight ratio and strain required for soft robot manipulators. OctArm is divided into three sections.

Each section is capable of two axis bending and extension which allows nine degrees of freedom. The manipulators are actuated with pressurized air (Maximum pressure = 120 psi) pressure control valves and polyurethane connective tubing. The air muscle actuators are optimized to provide the desired wrap angles and workspace. The distal section of each OctArm is designed to have a minimum wrap diameter of 10 cm.

The length of each section is chosen so that the manipulator can provide a range of 360 degrees wrap angles to accommodate a wide range of objects sizes. To provide the desired dexterity, OctArm is constructed with high strain extensor actuators extend up to 80%. To provide two-axis bending and extension, three control channels are used.

Six actuators are used in sections one and two and three actuators are used in section three. The six sections have two actuators for each control channel and results in actuators located at a larger radius, corresponding to higher stiffness and load capacity. Secondary layers of mesh sleeving are used to group individual actuators in control channels. Three closely-spaced actuators provide high curvature for the distal sections.

The third, visible, mesh layer or fabric skin is designed to protect the manipulator from abrasion and wear. For the field tests, OctArm was mounted to the second link of a Foster-Miller TALON platform. The control valves and two air tanks provided nine channels of controlled pneumatic pressure. Clemson University provided the control electronics and operation interface for these tests. The OctArm /Talon system underwent extensive field trials in the spring of 2005 at the Southwest Research Institute (SwRI) in San Antonio, Texas. (ed note: has links to some PDF reports) •.

(ed note: the summarizes the plot thusly: 'A robot designer struggles to understand why the current generation of robots is so inefficient. Eventually, he realizes that the human form (of current generations of robots) is a.' In the novel, the Empire wants to negotiate a mining treaty with the Martians. Who have tentacles, by the way. The problem is that the key Martian ambassador is partial to a cocktail called a 'Three Planets'. Only a Martian bartender can make a proper Three Planets, something to do with using tentacles.

Our Heroes are contracted to make a robot bartender capable of mixing a proper Three Planets. This is a problem, since if you add three drops of vuzd liquor to the drink it is incipid, but if you add four drops it tastes nasty. Our Heroes enlist the aid of a Martian Bartender named Guzub.) 'I got one of those new electronic cameras — you know, one thousand exposures per second So we took pictures of Guzub making a Three Planets, and I could construct this one to do it exactly right down to the thousandth of a second.

The proper proportion of vuzd, in case you're interested, works out to three-point-six-five-four-seven-eight-two-three drops. It's done with a flip of the third joint of the tentacle on the down beat. 'It didn't seem right to use Guzub to make a robot that would compete with him and probably drive him out of business, so we've promised him a generous pension from the royalties on usuform barkeeps.' I took one sip and said, 'Where's Guzub? This Three Planets, it's perfect.' Quinby opened a door.

There sat the first original Quinby usuform — no remake of a Robinc model, but a brand-new creation. Quinby said, 'Three Planets,' and he went into action. He had tentacles, and the motions were exactly like Guzub's except that he himself was the shaker. He poured the liquids into his maw, joggled about, and then poured them out of a hollow hoselike tentacle. (ed note: ' means a robot that is designed along functional lines, instead of stupidly forcing the design to look like a mental man). These are critters that look like large quartz crystals, often with flashing lights inside. Most are immobile, some can move.

Some crystal life is, other are not. An odd one was the. They were not invading aliens so much as an extraterrestrial chemical reaction.

Instant monster: just add water. In some cases the line between crystal life and is very blurry. The most obvious basis for such life is that it is based on semiconductor electronic circuits that somehow evolve and become more complicated inside the crystals. (ed note: Mr.

Miller apparently got a good bargain on a crate of exclamation points.) It wakened a lustrous opalescence in the two great spheres (alien spaceships) that nestled like mighty twin pearls against the dark rock, to create beings of the rock and of the shadow, gliding wraithlike among the shattered boulders! Painfully I crept through the dense growth of the brink, nearer to those great spheres and their dreadful cargo.

Within me my brain whirled and throbbed, my throat froze against the cry of shocked incredulity that rushed to my lips, cold, clammy sweat oozed from gaping pores! It was beyond all reason — all possibility! And yet — it was! Now I could see them clearly, rank on rank of them in orderly file, some hundred of them, strewn in great concentric rings about the softly glowing spheres — harsh as the black rock itself, hard, and glittering, and angular — a man’s height and more from summit to base — great, glittering tetrahedra — tetrahedra of terror! They were tetrahedra, and they were alive — living even as you and I! They stirred restlessly in their great circles, uneasy in the dim light. Here and there little groups formed, and sometimes they clicked together in still other monstrous geometric shapes, yet always they moved with an uncanny stillness, darting with utter sureness among the scattered rocks.

And now from the nearer of the twin spheres came another of their kind, yet twice their size, the pearly walls opening and closing as by thought-magic for his passing! He swept forward a little, into the full light of the moon, and the rings followed him, centered about him, until the spheres lay beyond the outermost and the giant tetrahedron faced alone the hosts of his lesser fellows! Then came their speech — of all things the most mind-wracking! I felt it deep within my brain, before I sensed it externally, a dull, heavy rhythm of insistent throbbing, beating at my temples and throwing up a dull red haze before my staring eyes!

“Yes, I’m Hawkins. The plane is somewhere over there, if it didn’t burn, with all your supplies in it. I was held up crossing the mountains. But tell me, first — those things, there — are they alive?” “You've wondered that?

I suppose anyone would. The Indians make them gods of a kind — realize they’re beyond all experience and tradition. But I'm a biologist.

I have had some experience in strange forms of life. They are as much alive as we — perhaps even more than we.

After all, if life is energy, why should it not rest where it will? Need we — soft, puny things of carbon and water and a few unstable elements — be the only things to harbor life? But this is no place to moralize — come on!” •. And through the curtain where fire of heavens and fire of Earth met in that terrible holocaust, those three saw the curving flames of the twin spheres gape wide, saw huge angular shapes file from the darkness within — shapes never yet associated in the Mind of Man with the meaning of life! Careless of the flame that seethed about them, they glided out over the fusing rock of the valley floor, score on score of them, showing in the fierce glare as mighty, eight-foot tetrahedra of dark, glistening crystal. They were of a purple that seemed to be of the essence of the things themselves, rather than a pigmentation of their surface; and near one apex each had two green-yellow unstaring, unseeing eyes!

Within them one glimpsed a spherical body — purple too — from which ran hundreds of curious filaments to the smooth surfaces. Tetrahedra they were — living tetrahedra of chilling terror that feared neither flame nor lightning and spread destruction on every side!

Sick at heart the three men watched, while the flames died and the winds came and stripped the blanket of dust and ash from the blasted rock. The tetrahedra meanwhile glided about their endless affairs, forming and reforming in geometric pattern. Or they clicked swiftly into many-faceted forms that in turn mounted into monolithic, crystalline monstrosities, then melted with startling suddenness into their original components. These were idle, pointless maneuverings from the human viewpoint, yet fraught with some hidden meaning and purpose as alien to Earth as the things themselves.

They suggested the terrible energies that were under their control — energies such as our little science has never hinted. Now, in the full light of day, I could see that it was as Professor Hornby had said. The tetrahedra were formed from some hard, crystalline mineral, black almost to invisibility, with a faint wash of rich purple running through it.

As they moved, the sun sent up glittering flashes of brilliance from their polished flanks, dancing like little searchlight rays along the shadowed face of the forest. For the tetrahedra were restless, were weaving aimlessly in and out among the boulders in weird arabesques as of some unearthly dance of the crystal folk, were condensing in little groups of half a dozen or less that formed and broke again even as do restless humans, waiting impatiently for some anticipated event. Apart from the rest, motionless in a sort of circular clearing among the rocks, squatted the giant leader of the tetrahedra. In him the deep violet of the crystal became a rich, plum-like hue, purple flushed with warm red, and the underlying black seemed less harsh.

It was warmer and more like the calm velvet of the tropic night. But these are impressions, qualitative terms with which to distinguish him in some way other than by mere size from his fellows. To an observer, the distinction was apparent, but it is not easy to express in everyday terms. It must suffice that he was indefinably different from the others, that he seemed to have character and personality, where the rest were but pyramidal crystals, albeit terribly alive. And now the giant leader was dinning out his mighty call in long, slow billows of beating sound that seemed to thrust me back, press me into the dark of the forest, away from the alien monsters of the valley! In response came thirty of the lesser tetrahedra, chosen seemingly at random from the scattered ranks, to range themselves at equal intervals about their master, forming a single great circle a dozen yards in diameter. Again the throbbing call shattered against the cliffs about me, and now all the hordes of the tetrahedra broke into flowing motion, converging in a torrent of glittering purple crystal upon the natural amphitheater, clustering in threes at the spots that their fellows had marked — all but ten, who glided into place before every third group, forming a giant toothed wheel with hub and rim and spokes of living, sentient crystal — crystal with a purpose!

There under that blazing sun they lay, gleaming like giant purple gems against the jetty rock. I thought of the great stone wheel of Stonehenge, and of the other monolithic circles that men have found in England and on the Continent. Strange resemblance, between the pattern of living monsters of another world and the ancient temples of a prehistoric race! And yet, is it too far-fetched to suggest that the superstitious savages should pattern their greatest temples after the unearthly gods of their worship — gods of purple crystal that came and smote and vanished again into the skies, leaving the memory of their inevitable circling, and the thunder of their language in the great drums of worship? May it not be that they have come before, and found Earth unfitted for their usage, and passed on to other worlds?

And if they have so come, and found us wanting, what lies beyond that has prevented them from bearing back the tale of their findings, marking Earth as useless for their tetrahedral purposes? Why have they had to come again and again? I COULD see that the groups of three that formed the toothed rim of the giant crystal wheel were tipping inward, bringing their peaks together in a narrow focus, and more, that the ten that were the spokes, the binding members of the wheel, were of the same rich hue as their master. The shadows of the myriad tetrahedra squatted short and black about their shining bases, against the shining rock. As the sun soared higher, pouring its blazing rays straight down upon the sweltering world, I sensed the beginning of a vague roseate glow at the foci of the circling trios, a glow as of energy, light, focussed by the tetrahedra themselves, yet not of themselves, but sucked from the flood of light that poured upon them from above. For the light that was reflected from their sides gleamed ever bluer, ever colder, as they drank in the warm red rays and spewed them forth again into the seething globes of leashed energy that were forming just beyond their pointing tips! The rose-glow had deepened to angry vermillion, seemingly caged within the spheres defined by the tips of the tilted tetrahedra.

Thirty glowing coals against the black, ninety great angular forms gleaming ghastly blue in the pillaged sunlight, forms that were slowly closing in upon the center, upon their mighty master, bearing him food, energy of the sun for his feasting! Now the scarlet flame of the prisoned light was mounting swiftly in an awful pinnacle of outrageous color — pure fire torn from the warm rays of the sun — raw energy for the glutting of these tetrahedral demons of another world! It seemed to me that it must needs burst its bounding spheres and fuse all that crystal horde with its unleashed fury of living flame, must win free of the unimaginable forces that held it there between the eager, glittering facets, must burst its unnatural bonds and sweep the valley with a tempest of awful fire that would consign the furnace of the tetrahedra to pitiful insignificance!

It did none of these, for the power that had reft it from the golden sunbeams could mould it to the use and will of the tetrahedra, as clay before the potter! Slowly the great ring contracted, slowly the tetrahedra tipped toward their common center, bearing at their foci the globes of angry flame.

Now they stopped, hung for a long moment in preparation. Then in an instant they loosed the cradled energy of the spheres in one mighty blaze of blinding crimson that swept out in a single huge sheet of flame, blanketing all the giant wheel with its glory, then rushing into the blazing vortex of its center.

Here, all the freed energy of the flame was flowing into the body of the mighty ruler of the tetrahedra, bathing him in a fury of crimson light that sank into his glowing facets as water into parched sand of the desert, bringing a fresh, new glow of renewed life to his giant frame! And now, as in recoil, there spouted from his towering peak a fine, thin fountain of pale blue fire, soundless, like the blaze of man-made lightning between two mightily energized electrodes — the blue of electric fire — the seepage of the giant’s feast! Like slaves snatching at the crumbs from their master’s board, the ten lesser tetrahedra crowded close. As their fierce hunger voiced itself in awful, yearning force, the fountain of blue flame split into ten thin tongues, barely visible against the black rock, that bent down into the pinnacles of the ten and poured through them into the crowding rim of the giant wheel, a rim where again the spheres of crimson fire were mounting to their climactic burst! Again the crimson orbs shattered and swept over the horde in a titanic canopy of flame, and again the giant master drank in its fiery glory!

Now the fountain of seepage had become a mighty geyser of sparkling sapphire light that hurtled a hundred feet into the shimmering atmosphere, and, bent by the fierce hungering of the lesser creatures, curved in a glorious parabola above the crystal wheel, down over them and into them, renewing their substance and their life! For as I watched, each tetrahedron began to swell, visibly, creeping in horrid slow growth to a magnitude very little less than that of their giant leader.

And as they mounted in size, the torrent of blue fire paled and died, leaving them glutted and expectant of the final stage! It came, with startling suddenness! In an instant each of the hundred clustering monsters budded, burst, shattered into four of half its size that cleaved from each corner of the parent tetrahedron. They left an octahedral shape of transparent crystal, colorless and fragile, whence every evidence of life had been withdrawn into the new-born things — a shell that crumpled and fell in fine, sparkling crystal dust to the valley floor. Only the giant ruler lay unchanged beneath the downward slanting rays of the sun. The hundred had become four hundred!

The tetrahedra had spawned! Four hundred of the monstrous things where a hundred had lain the moment before! Drinking in the light of the noonday sun, sucking up its energy to give them substance, these tetrahedral beings from an alien world held it in their power to smother out the slightest opposition by sheer force of ever-mounting numbers! Against a hundred, or four hundred, the armies and the science of mankind might have waged war with some possibility of success, but when each creature of these invulnerable hosts might become four, with the passing of each noon’s sun, surely hope lay dead! Man was doomed! “Do you realize that this spawning means that they’re ready to go ahead and burn their way right through everything — make this whole planet a safer and better place for tetrahedra? Doc has figured they’re from Mercury — overcrowded, probably, by this wholesale system of reproduction in job-lots, and hunting for new stamping-grounds.

I don’t know what our chances are of bucking them — about a quarter of what they were an hour ago — but they’re mighty slim, armed as we are. You’ve got the other machine-gun?” I HAD no trouble in finding the Professor.

In truth, he found me. He was all but boiling over with excitement, for he had seen something we had not. “Hawkins,” he exclaimed, grabbing my shoulder fiercely, “did you see them spawn?

It is remarkable — absolutely unequalled! The speed of it all — and, Hawkins, they do not have to grow before cleaving. I saw two that divided and redivided into three-inch tetrahedra — over a thousand of them! Think of it — Hawkins, they can overrun our little planet in a few days, once they start! We’re done for!” Now, their army of destruction assembled, the tetrahedra began their conquest of Earth! In vast waves of horrid destruction with rays of angry yellow flame darting from apexes their flaming floods of energy swept over the jungle, and now not even its damp dark could resist. Mighty forest-giants toppled headlong, by the cleaving yellow flame, to melt into powdery ash before they touched the ground.

Giant lianas writhed like tortured serpents as their juices were vaporized by the awful heat, then dropped away in death to lie in long grey coils along the stripped rock of the forest floor — rock that was fast taking on the glassy glare of the little valley, rock fused by heat such as Earth had never known. Now we could watch their plan of campaign, and our hearts sank in fear for our race, for while half of the tetrahedral army engaged in its holocaust of destruction, the remaining half fed and spawned in the full blaze of the sun. With every day dozens of square miles were added to their hellish domain and thousands of tetrahedra to their unnatural army. For now we could see that more and more of them were taking the second course, were splitting into hosts of tiny, three-inch creatures which, within a few days’ time, had swelled to full size and on the following day could spawn anew! The yelling circle was thinning fast, yet they had not realized the futility of their attack when suddenly the tetrahedra deserted quiet defense for active combat! The cause was evident.

Five Indians on the upslope had shoved over the cliff a huge rounded boulder that bounded like a live thing among the rocks and crashed fufl into the side of a great eight-foot tetrahedron, splintering its flinty flank and freeing the pent-up energy in a blinding torrent of blue flame that cascaded over the nearby ledges, fusing them into a white-hot, smoking pool of molten lava that glowed evilly in the ill-lit gloom! It was the last straw! The mad attack had become a thing of real menace to the tetrahedra, and they sprang into swift retribution.

From their apexes they flashed out the flaming yellow streaks of destruction. Ever since Marston had first mentioned Professor Hornby’s theory that the things were Mercutians, I had been trying to find some way of verifying it. Now that we were in semi-intimate terms with the tetrahedra, I wondered if I might not get them, somehow, to supply this evidence. I thought of stories I had read of interplanetary communication — of telepathy, of word-association, of sign-language.

They had all seemed far-fetched to me, impossible of attainment, but I resolved to try my hand at the last. There was some rather soft rock in the structure of the watch-tower, and as Valdez had rescued my tool kit from the plane, I had a hammer and chisel. With these, and a faulty memory, I set out to make a rough scale diagram of the inner planets, leaning a bit on the Professor’s theory. I cut circular grooves for the orbits of the four minor planets — Mercury, Venus, Earth, Mars — and dug a deep central pit. In this I set a large nugget of gold, found in the ruins of the fortress, for the Sun, and in the grooves a tiny black pebble for Mercury, a large white one for Venus, and a jade bead from the ruins for Earth.

Earth had a very small white moon, in its own deep-cut spiral orbit. Mars was a small chunk of rusty iron with two grains of sand for moons. I had a fair-sized scale, and there was no room for more. Now I was prepared to attempt communication with the tetrahedra, but I wanted more than one diagram to work with. Consequently I attempted a map of Earth, with hollowed oceans and low mountain-ridges. A cloud-burst, it would be called in the United States. The heavens opened in the night, and water fell in torrents, streaming from every angle of the rock, standing in pools wherever a hollow offered itself, drenching us and the world through and through.

Day came, but there was no sun for the tetrahedra to feed on. Nor were they thinking of feeding, for very definite peril threatened them. To the tetrahedra, water was death! As I have said, their fires had flaked huge slabs of rock from the walls of the ravine leading from the high-walled valley where they slept, choking its narrow throat with shattered stone. And now that the mountain slopes, shorn of soil and vegetation, were pouring water into its bed, the stream that had carved that ravine found its course dammed — rose against it, poured over it, but not until the valley had become a lake, a lake where only the two pearly spheres floated against the rocky wall, the thousands of tetrahedra gone forever — dissolved! Water was death to them — dissolution! Only in the shelter of the spheres was there safety, and they were long since crowded.

The hordes of the tetrahedral monsters perished miserably in the night, before they could summon the forces that might have spun them a fiery canopy of arching lightnings that would drive the water back in vapor and keep them safely dry beneath. A hundred had come in the twin spheres. A hundred thousand had been born. A bare hundred remained. (ed note: Our heroes use the the map of the solar system to explain to the tetrahedrons that [a] water is death to tetrahedrons, [b] Earth is 75% water, [c] right now Earth is in the dry season. Implication is that if the cloud-burst that killed 99.9% of their invasion force happens in the dry season, the wet season will be utterly deadly. Perhaps it would make more sense to go invade Mars?) •.