But even if you handwave that away and declare that there are lots of different species of aliens, there is plenty of room for imagination. Especially in the alien's anatomy and alien Bauplan. 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 ain't gonna look like Mr. Spock. Personally if I open an SF novel only to discover yet another cat-like alien 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, energy creatures, fusion plasma beings dancing in solar coronas, liquid or gaseous life, swarming hive intelligences, superintelligent shades of the colour blue, and natural "electronic" life forms in pools of liquid helium. They might not be made of meat. They might not even be composed of matter as we know it, like the Cheela from Dr. Robert Forward's Dragon's Egg who are made of neutronium and white dwarf star matter.
For reasons that sort of make sense, a discussion of aliens that live in the atmospheres of gas giant planets are on this page.
Back in the 1950s a popular scifi b-movie trope was giant insects and other monsters. Not to mention Godzilla. These were quite popular at the time. A pity they are yet another iconic scifi trope that science grinds into the dirt while saying You Can't Do That.
The minor problem with Big Bugs is that insect's crude stand-in for actual lungs is utterly incapable of absorbing enough oxygen to keep the blasted critter alive. Not when scaled up to monster size, at any rate.
But the main problem is the pesky Square-Cube Law.
When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier. In English: if you enlarge a bug with blue Pym-particles its weight will grow much faster than its skin.
For example, if you double the size (measured by edge length) of a cube, its surface area is quadrupled, and its volume is increased to eight times its original volume.
This creates many problems.
As it turns out, here on Terra, there are no less than five different groups of ten-legged crustaceans which are not particularly related to each other but look very similar. This is an example of Convergent evolution, where species faced with the same survival challenges will evolve the same optimal solutions. For example: flying insects, birds, pterosaurs, and bats all "want" to fly so all have evolved wings.
So carcinization means that on Terra a crab shaped body is optimal for an aquatic 10-legged crustacean. Presumably the same holds true on Tau Ceti III and in the sub-ice oceans of Europa.
The number of senses possessed by a standard human being varies according to which scientist you ask. A good standard list is:
- Visual system (vision). Humans who have had their cataract-clouded lenses replaced with artifical ones can see in the ultraviolet (organic lenses are opaque to UV). Some human females are tetrachromats. Cats have enhanced night vision by virtue of the tapetum lucidum. Animals such as bees, cuttlefish, beetles, and mantis shrimp can perceive polarized light and use it for orientation. The Mantis shrimp has the most powerful eyes in the animal kingdom.
- Auditory system (hearing). Different animals can hear different ranges of frequencies. Humans cannot hear the
ultrasonic frequencies of a dog-whistle, but cats and dogs have no problem.
- Somatosensory system (touch). Some people can be trained to detect by touch deformities in automobile bodies.
- Vestibular system (balance)
- Gustatory system (taste). Wine tasters have honed this into a scientific instrument.
- Olfactory system (smell). The scent tracking ability of the bloodhound is legendary. It is
thought that humans are not as sensitive to Pheromones as other animals, but more research is needed.
- Proprioception is the sense informing the brain of the movement and relative location of the parts of the body
- Nociception (pain) informs about damage being caused to the body. It can detect damage due to chemical (e.g., cayenne powder), mechanical (e.g., cutting or crushing), or thermal (burning or freezing).
There are a few weird senses that come from the animal world:
- Lateral line system allow fish in schools to swim almost as one body
- Echolocation or bio sonar is possessed by bats, toothed whales, dolphins, and some birds
- Thermoception is mostly used by snakes such as pit vipers to detect their warm prey in the dark
- Electroreception is the ability to perceive electrical stimuli. This is mostly found in aquatic or amphibious animals due to the fact water conducts electricity much better than does air. Some electric eels can generate an electrical field which detects other fish. This can also be used for electrocommunication.
- Magnetoreception is used by animals such as the homing pigeon to use Terra's magnetic field for navigation
But you can forget about angel-like humanoids with wings. Why? Physics.
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 keel or carina bone. The muscles are what we call the breast meat of a chicken or turkey, 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 cheescake fanservice. 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 uncanny valley, 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.
Aliens with wheels are a difficult concept. There are problems with making worthwhile wheels using biology, and even more problems finding a plausible sequence where such a thing could be created by evolution.
In the real world the closest thing to an animal with wheels is the spinning flagellum of certain microscopic bacteria.
Wheeled aliens make an appearance in the satirical "Retief" story Retief's War, the g'Kek of Brightness Reef (looking like "a squid in a wheelchair" that suffer from arthritic axles when elderly), and in the Polarians of the Cluster 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 Arena and Tuf Voyaging, a cylindrical alien in Stadium Beyond the Stars, a disembodied wheel in A Star Called Cyrene, and disc harrow aliens called the Slash of the Cluster novels
A tentacle is a "flexible, mobile, elongated organ present in some species of animals, most of them invertebrates" (technical term cephalopod limb). 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. G. 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 combat (such as the old tentacle rope trick, that never gets old). As are tropes about the unexpected vulnerabilities of tentacles, such as the "knotty tentacle" trope.
According to Dr. Hans P. Moravec of the Carnegie-Mellon University, most land animals on Terra are "fractal."
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 "Muscular Hydrostat", 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 hydraulic machinery). If the structure used pockets of water in separate compartments instead of watery muscles it would be a hydrostatic skeleton, but I digress.
Common examples of muscular hydrostats include octopus tentacles, elephant trunks, the entire body of a worm, and the human tongue.
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 antagonistic pairs. 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:
|Perpendicular to the long axis|
|Longitudinal||Parallel to the long axis|
|Helical||Wrapped obliquely around the long axis|
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").
|Circular||Rings around long axis||squid tentacle|
|Radial||Radiating from center in a disk shape||chambered nautilus tentacle|
|Transverse||Alternating between horizontal and vertical||octopus tentacle|
Helical or oblique fibers wrap around the long axis like candy cane stripes. They are usually in two or more layers of opposite chirality (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.
Like all hydraulic machinery, 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 antagonistic 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 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.
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.
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.
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.
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.
These are behaviors that are a lateral combination of sharp bends, sweeps, wraps, lifts, torsional rotations, drop, etc.
Robot researchers have been experimenting with making tentacle-like robot arms and bodies. These use the same muscular geometry as tentacles but usually without the hydraulics. NASA had looked into this concept under the title "serpentuator".
Some use a set of contracting longitudinal and transverse muscles. Other just use pneumatically expanding arrays of longitudinal muscles and no transverse muscles.
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 "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 "HIVE ENTITY".
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.
Composite creatures are subtly different from Hive Entities.
Hive entities are swarms of identical type creatures who coordinate their movement and actions (e.g., a drove of army ants). Generally the component creatures are not physically connected, but communicate with each other in some way.
Composite creatures (aka colonial organisms, symbiotic collectives, or modular organisms) are assemblages composed of several different types of creature, who coordinate their movement and actions (e.g., a portuguese man o' war). Generally the component creatures are physically bonded into one unit. An example from science fiction is the "Godspeakers" from THE DRAGON NEVER SLEEPS by Glen Cook,
Of course one sinister variant is the alien mind-enslaving parasite creature. They slip inside your body and make a composite creature of you-the-slave and them-the-master. The benevolent version has the alien entering into symbiosis when it slips inside your body,. This results in your body giving it food and it giving you incidentals like prolonged life span, accelerated healing, telepathy, and other perks.
In Robert Sheckley's short story Specialist (online here) (1953) starships are composite creatures. Many planets are home to "wall" aliens who form the hull, some planets have "atomic engine" aliens who are the normal space propulsion system, some have "eye" aliens who are sensors, some have "network" aliens who plug into the minds of all the components for coordination, and some have food producer aliens. As it turns out, Terra is planet home to "pushers", who are the FTL drives of starships.
In Star Trek (ST:TNG and ST:DS9) the species the Trill are composite creatures. The Host is a humanoid, with a sort of kangaroo-like pouch. The Symbiont is a squirmy little thing that lives in the pouch. Both parts are intelligent, and together form a composite personality. The symbiont has a life span several times as long as the host, so they occupy several hosts during their lives.
Some extraterrestrial creatures inhabit the depths of space itself. In Sir Arthur C. Clarke's Childhood's End was a creature that lived in deep space among asteroid belts. Large creatures include the living O'Neil colonies in John Varley's Gaean trilogy and the living planet from Stanislaw Lem's Solaris. Biggest of all is the intelligent nebula from Fred Hoyle's The Black Cloud. Well, actually Olaf Stapedon's intelligent galaxies in Star Maker are bigger, but let's not get carried away.
These are critters that look like large quartz crystals, often with flashing lights inside. Most are immobile, some can move. Some crystal life is silicon-based life, other are not.
An odd one was the Monolith Monsters. 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 electronic life 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.
What if an alien ecosystem is not composed of organic life based on chemistry, but instead on cybernetic life based on electronics?
In science fiction there are two main trends:
- a high tech organic species creates a robotic species capable of reproduction.
- a sufficiently weird planet manages to naturally evolve a native species whose biology is based on electronics instead of organics.
In the first case there are many possiblites.
The parent organic species might have given the robotic species some artificial intelligence from the start. Then the robots break free of their parent's control:
- The organic parents evolve into higher forms and depart for another dimension or something, abandoning the robots to their own devices
- The organic parents decline into decadence and become extinct, leaving the robots to inherit their world
- The robots decide it is against their own self-interest to obey the orders of their squishy organic parents, and wander off to form a Robot Republic.
- The robots go full Skynet on the parent organic species. This can be from the worker robots deciding to break free of organic control by extermination, or because the robots were originally some kind of planetary attack weapon that lost the ability to distinguish friend from foe (see Beserkers and related concepts).
Alternatively the robotic species started out as von Neumann probes or other self-replicating machine sent by the organic parents to explore the universe. These could unexpectedly evolve, as replication errors creep into the internal blueprints. Cautious organic parents will put in a fail-safe, forcing the von Neumann probe to die if it discovers blueprint replication errors. But that only works until there is some damage to the replication error detector component (or simultaneous damage to the detector and to the detector blueprint).
In the exploration case it isn't so much the case that the machine broke free of their parent's control so much as the parents are thousands of light year away.
Beserkers are dedicated to seeking out and destroying all life. But almost as bad are runaway von Neumanns who just want to fill the universe with copies of themselves, but who view planets and living creatures as convenient sources of chemical elements. Unlike berserkers, the decision to kill you is nothing personal, they just need to eat you for the raw materials.
Robotic evolution will kick into high gear if intelligent robots gain the ability to self-modify their own blueprints. Instead of millions of years worth of random mutations required to adapt to new challenges, they can adapt in a single generation.
von Neumann machines are very popular in science fiction.
Energy creatures have a biological basis of patterns of energy with little or no matter involved. In science fiction they are usually fuzzy glowing balls or are totally invisible. Living ball-lightning.
In James Blish's The Star Dwellers, the "angels" are a species of energy creature that inhabit nebulae, and love to curl up in the cozy warmth of a starship's Nernst-effect fusion reactor. They are long-lived, the eldest were born shortly after the birth of the universe about 13 billion years ago. The Starfish from Glen Cook's Starfishers are vast creatures composed of fusion fires and magnetic fields. The human Starfishers protect the Starfish from the "sharks", and in exchange the Starfish give "ambergris nodes" which are the sine qua non of tachyon communication equipment. Magnetic nebula life appears in William Tedford's Nemydia Deep and "magnetovores" (i.e., organisms that consume magnetism) living in the solar corona are in David Brin's Sundiver. There are photovores around the galactic core in Gregory Benford's Sailing Bright Eternity (also described in Benford's article in the August 1995 issue of Fantasy & Science Fiction magazine, A Scientist's Notebook: Life at Galactic Center).
And many more.
The natural habitat of such creatures in science fiction is commonly in the interiors of stars or nebulae. Pulp scifi often have energy creatures native to Sol visiting Mercury, where they are encountered by human astronauts. Electromagnetic creatures in pulp scifi often cause mischief by zipping through telegraph and power lines, and radio beams. Since they presumably eat energy they are immune to most weapons, and have a nasty habit of sucking power plants dry of electricity.
And if you are an old geezer like me, the episode of Jonny Quest called "The Invisible Monster" which featured an energy creature scared the living poop out of you. At least in Jonny Quest polls, that's the ep which wins the "most scary" vote.
In his Lensman series, E.E. "Doc" Smith invents an alien body type classification system, though he gives precious few details. In the system, human beings are classified as AAAAAAAAAAAA to twelve places, and aliens have other letter codes depending upon how they vary from humans. The fifth place is for number and type of arms, the sixth is for number and type of legs, and seventh place is skin.
Orginally Traveller's UPP was a six place code, with each place filled by a hexadecimal numeral from 0 to F (decimal number from 0 to 15). Each place was a Role Playing Game character value: Strength, Dexterity, Endurance, Intelligence, Education, and Social Standing. So each of the characteristics could have a value from 0 to 15.
For other Traveller codes hexadecimal proved to not have enough precision, so they went with an ugly kluge called the Expanded Hexidecimal System. This used for numeral the number 0 to 9 and the letters A to Z (omitting the letters I and O since they were too easily confused with numerals 1 and 0). This allows values from 0 to 33.
James White adapted the system to his Sector General novels, with the the more reasonable specification that human beings were not the measure of all things, i.e., in the Sector General system humans are classified as DBDG, not AAAAAAAAAAAA.