Xenomorph

"You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility."

- Ash

The Xenomorph XX121, better known just as Xenomorph (which literally translates to "strange form")or the Alien, are an extraterrestrial, endoparasitoid species with multiple life cycles, possibly originating from the planet Proteus (also known as Xenomorph Prime). One of the most deadly of all known alien species, these creatures need a host organism in order to reproduce. The appearance of the Xenomorph varies depending on its host. The human phenotype is generally around 7–8 feet, and roughly 136.0 to 181.4 kilograms with a long, muscular tail and large, curved, oblong head. The Queen of this species is generally twice as large (they can grow even larger, some even up to 100 feet tall, and stronger if given time) and possesses superior speed, strength and intelligence. The term Xenomorph is derived from the Greek words xeno ("stranger", "alien", and "foreigner") and morphe ("form", "shape").

History
There are no solid facts as to the origins of the Xenomorph species; instead, there are many assumptions which cannot be confirmed. Based on the limited information we have, the most commonly accepted hypothesis is that they are an artificially created species, although another hypothesis says that they evolved naturally on a planet much different than our own.

The Xenomorph could be the result of genetic manipulation by Mala'kak (also known as the "Engineers" or "Space Jockeys") as a terraforming mechanism about ten million years ago, but apparently it went horribly wrong. As a result, over the years the Xenomorph has become an entirely new species in the galaxy. This theory has been twisted slightly by Prometheus, when the Mala'kak brought containers filled with an unknown substance (it closely resembles Xenomorph secretions in hives). This substance, mutated harmless organisms into somewhat hostile creatures. When humans "infected" with this substance, mate with one another it produced a different subspecies of "octo-facehuggers" that can grow incredibly large. As shown in the movie, this new subspecies regardless of who it infected, Mala'kak or human, would result in the "Deacon" or ''Proto-alien. ''It is not known what new subspecies would result if the "octo-facehugger" impregnates a Yautja.

Another theory, commonly accepted, touched on in the expanded universe is they were the alpha predator of their own ecosystem on a nightmarish and harsh planet (some assume it to be Xenomorph Prime, but there is no solid evidence). Without their native ecosystem to keep them in check, they have since infested their own kind, and developed into a separate dominate species. Other parts of the universe began to take notice in this interesting species and began gathering them. The Mala'kak, the Yautja, Humans, and an unspecified highly technological humanoid species introduced in the comic Alien: Reaper began using them as means of sport, terraforming mechanism, as biological weapons, food delicacies, and scientific experiments.

One interesting idea though, is that they were brought over by the Mala'kak as biological weapons against an unknown threat (such as the Yautja species) or a weapon to wipe out humanity and other species that exist in the galaxy. The eggs at the derelict ship found at LV-426 was in fact a biological weapon that turned against the crew. Ridley Scott states during the Director's Commentary on the Alien Special Edition DVD that the eggs are the cargo of the Space Jockey's ship with the ship being a war-like transport ship designed to carry these biological weapons, or perhaps a science vessel carrying the eggs as cargo for scientific study. This incident allowed humans to take more notice into account about the Xenomorphs that they do know exist in the galaxy.

Biology
The Xenomorph physical form varies wildly depending on its life stage, hive caste, age and host, with numerous genetic varieties existing whose origins are completely unknown, as well as varieties which have been genetically engineered by host species or genetically grown. The typical Xenomorph starts life as an egg-like creature which, when detecting the presence of a potential host, opens up to release the second stage: a small quasi-arachnoid life form which launches itself onto the host's face and impregnates it with an endoparasitoid larva, which later consumes the host from within and exits the body as a vaguely worm-like organism, which then grows very quickly, developing into the adult Xenomorph.

Although numerous varieties exist, the adult Xenomorphs all tend to share some common traits: a vertebrate-like body, a protective exoskeleton, a long flexible tail, an elongated eyeless head, and a toothed maw which hides a retractable inner mouth. Little is known about their biology, but it is known that they eat with their "tongue mouths", not their outer mouths. This mouth is somewhat similar to the pharyngeal jaw of a moray eel. They also bleed extremely acidic fluid which is yellowish green in color and often referred to as "blood", although whether it's used to transport nutrients through the body and/or serves other functions is not clear.

While the extremely acidic fluids and tremendous resilience displayed by the creature might suggest an alternative kind of biochemistry, the fact that the Xenomorph not only feeds on Humans but actually develops inside them strongly suggests that its biology is carbon-based. The ability to inherit traits from its hosts also suggests that it incorporates genetic material from them, implying that the Xenomorph either has DNA, or is able to read and convert DNA into whatever means it uses to store genetic information.

Intelligence
Xenomorphs appear to possess an roughly similar to that of a typical primate, but have shown to learn relatively fast and utilizing the situation around their environment. Although they do not demonstrate Human-level intelligence as a species (such as abstract reasoning, introspection, and technological advancement), their queen appears to possess considerable acumen in its social behavior and manipulation of human technology, such as cutting the power of the LV-426 colony and operating an elevator. In the director's commentary for Aliens, James Cameron noted that the Xenomorphs in Aliens had been alive far longer than the Xenomorph in the original, and so has had more time to learn how to manipulate machinery, if only at the most basic level. However, the genetic Xenomorphs from Alien: Resurrection learned very quickly that they were being contained, so they killed the weaker one so the acid of the Xenomorph's blood would seep through the floors, thus supporting the idea of fast learning.

As the adult Xenomorphs seem to inherit physical characteristics from their hosts (exemplified, for example, when a Yautja became infected, the parasite that later developed into a "predamorph" or "predalien") it is possible that the intelligence level of the creature may be influenced by its host as well. Furthermore, being a eusocial species, it is possible that the intellectual capacity varies for each different caste.

Xenomorphs have demonstrated little emotion, though they are not completely devoid of fear as shown in Aliens vs Predator: Requiem when the, especially for their eggs, or in the presence of fire. They have also been shown to exhibit pain through shrieks or screams when attacked.

In the Aliens literature, it has been suggested that the creatures may have a hive mind, or a collective consciousness directed by the Queen, who can call back and give orders to her warriors, telepathically and with shrieks. In the Aliens comic book series, as well some novelizations (produced before the Alien³ film), it is suggested that the Queen Mother Xenomorph communicates telepathically with its potential hosts, through terrifying dreams and religious visions, leading to the formation of cults with the Xenomorph as a god-like figure. This is similar to the Cthulhu cult in H. P. Lovecraft's The Call of Cthulhu. This can be seen in Alien 3, when Golic is induced by supposed "telepathic" messages from a captured Xenomorph. This ultimately leads him to release it and let it kill him. He called it "The Dragon", and viewed it as a god.

Through cloning in the events surrounding Alien: Resurrection (noted in the novelization), it appears that the Xenomorphs' hive mind includes a collective memory that passes along even at a genetic level. Through the tests conducted on Ellen Ripley, it is revealed that the memory passed from Ripley onto the new generation of Xenomorphs allows them to read and understand different languages. Events on the LV-426 colony and the USM Auriga show that the species excels at observational learning. Another case of observational learning from Alien: Resurrection is seen, after a Xenomorph has escaped its cell. It had learnt that the consequences of attacking the glass through which they were watched was a jet of extreme cold (possibly liquid nitrogen), triggered by a large red button on the console. When a crewman enters the now-empty cell to study the hole through which the Xenomorphs escaped, one of them enters the control room and activates the button using its inner jaw.

In the rare event that a Xenomorph finds itself under attack or otherwise vulnerable, it might display incredible creativity and cleverness. They are capable of moving quickly and silently in ventilation systems, despite their size, and will use such conduits for ambush, escape, or simply as an easy way to travel between areas.

Xenomorphs are conscious of the effects of their acidic blood, and will use it to their advantage - to break out of human-constructed confinement, or as a weapon. They show some military tactics (such as retreating when overwhelmed), and can find ways to get around certain problems (such as pushing the red button to freeze a soldier in liquid nitrogen). They know that the darkness is their ally, and break lights to create cover.

Anatomy
The Xenomorph Warrior is equipped to be the perfect killing machine. It possesses an exoskeleton much like that of a lobster, impenetrable to everything but the most powerful human firearms (12-gauge shotguns, Pulse Rifles, and stanchion launchers are quite effective). Ash claims the creature's skin is made from 'protein polysaccharides', so this would imply a substance similar to chitin. He also says the creature replaces its surface cells with polarized silicone, which is a very inert, heat resistant, and flexible polymer. This allows the creature great environmental resistance. However, they are vulnerable to sudden, massive changes in temperature, as shown in Alien 3, when the creature is destroyed by first immersion in molten lead and then sudden cooling with fire sprinklers, causing its silicone shell to shatter.

Attached at the ends of both of its arms are a set of long fingers and shiny, black, and extremely sharp claws capable of slashing through almost anything within its grasp (as seen in ALIEN books, such as Book 2: Nightmare Asylum). It has two pairs of finger-like digits and appears to have two opposable thumbs (on opposite sides of the hand) on each hand. Its tail is long, and can be used as a whip. Later movies show it to have ridges on top, and a serrated toxic tip at the end to lacerate enemies.

It stands around 7 feet tall, averaging in at anywhere between 140 kg and 180 kg; although size may vary wildly depending on the host chosen, as seen for instance with the "Jock-Xenomorph". Its mouth is filled with sharp teeth, but its main killing device is the inner jaw that shoots out to penetrate the victim's brain.

The inner jaw (also known as an "attack tongue") is a muscular appendage on the inside of the Xenomorph's mouth. All Xenomorphs have this trait. It consists of a cord of muscle with a small but extremely deadly mouth-like appendage on the end. This attack tongue is able to punch through almost anything, from bone to Predator armor.

Physical Abilities
The adult Xenomorph is a living weapon, noted for its ferocity and deadliness in any condition. Once fully matured, they have great physical strength and agility. They are masters of stealth; a favored method of acquiring prey is to wait in a dormant state until an appropriate victim strays near, and then drop down silently from behind or use its tail to impale whoever walks by. The prey is generally blind to the fact that a Xenomorph is present, due to its propensity to camouflage itself within its nest walls or the surrounding artificial environment given its bio-mechanical-like appearance. A Xenomorph will also use its pitch black carapace to its advantage, lurking in the shadows whilst waiting for prey to stray too close.

Xenomorphs are shown to be able to take extreme amounts of physical damage that would kill any Earth lifeform. An example of this is the Queen in Aliens survived a multi ton exo-suit falling on her from over 30 feet. Xenomorphs are capable of surviving with limbs shot off as seen in AVP3 the game and Aliens. Even stabbings and shots through the head (though total decapitation kills them) as seen in Alien vs Predator and its sequel are not guaranteed to kill Xenomorphs.

Adult Xenomorphs are quick and agile, and can clamber along ceilings and walls, a skill they exhibit freely whether they are evading others, attacking, or hunting. They can survive in extreme temperatures, are well-adapted to swimming, can respirate in harsh atmospheres, and can survive in vacuum for short lengths of time. Their movement tends to be silent, and they do not radiate heat as their exoskeletal temperature matches the ambient temperature. They also salivate profusely. This saliva is not acidic, though some Xenomorphs do have the ability to spit acid, which may come from their stomachs, a special gland somewhere in their throats, or acid pouches lining the head. This can be used to blind victims, much like a spitting cobra.

The creatures possess no visible eyes; Giger mandated this in his original design, because he felt that it made the creatures much more frightening if one could not tell they were looking at them. In the Alien vs. Predator series, the game implies that the Xenomorph sees using its attack tongue. In the original Alien film, the top of the creature's head was semi-transparent, with empty eye sockets of human appearance visible within. This element was dropped in later movies, but reused for the Predalien design, on the front portion of its skull.

In Aliens, the adult creatures are lacking the smooth carapace covering their heads. In the commentary for Aliens, it was speculated that this was part of the maturation of the creatures, as they had been alive far longer than the original Xenomorph. This maturation process involves the now adult Xenomorph shedding the smooth dome-like plate covering its head, leaving the skin underneath to be exposed. In Alien³, a fish-eye lens (which creates a perspective similar to that of a peephole) was used to depict the Xenomorph's sight. Whether this can be considered sight or just a filmed representation of the creatures' sensory perception is unknown.

However, in the novelization of the movie Alien, the creature is held mesmerized by a spinning green light for several minutes. Due to the absence of clearly visible eyes, it is possible that the creature uses echolocation to see its environment, much like bats. It may be for this reason that Xenomorphs hiss almost constantly. Though it may have the ability to see through an unknown structure, as they evolved differently from life on Earth. In the 2010 Alien vs. Predator game, it is mentioned on one of the Marine audio tapes that Xenomorphs have vision.

Xenomorphs may be able to detect their 'prey' through electro-reception. This is the method sharks use to detect even the most hidden prey. Creatures with the electro-reception sense can detect the electromagnetic field all animals produce allowing them to detect the creature's heart beat. This would explain how Xenomorphs always know where the humans are.

And, as depicted in the Aliens vs. Predator games, the Xenomorphs can also detect their prey using pheromones. Another theory is that the eyes of a Xenomorph are behind their black carapace and they see through it, similar to a one sided mirror (except in this case no one can see in, but the Xenomorph can see out). Some species of terrestrial fish such as the Barreleyes see through a transparent layer of skin; it is possible Xenomorph vision is similar.

Xenomorphs can also produce a thick, strong resin, which they use to build their hives and cocoon victims. Much like termites, they mix their viscous saliva with solids. It shows amazing heat and moisture-retaining qualities.

Vulnerabilities
Xenomorphs, in all stages of their life cycles, have been said to have vulnerabilities to heat, although this was only shown in a deleted scene from the Alien Special Edition DVD, in which Ripley searches for her missing shipmates in the bowels of what appears to be the engine/boiler room of the Nostromo. It is also mentioned in the game AVP 3 by a flamethrower-carrying marine: "Buggers don't like fire. This'll flush 'em out".

The only Xenomorph not vulnerable to fire is the Predalien. In the game Alien vs. Predator: Extinction, it secretes a layer over their body. That layer cannot be ignited, though they are visibly pained by temperature extremes such as liquid nitrogen.

A rapid succession of extremes in both high and low temperatures apparently causes a thermal shock effect on a Xenomorph's exoskeleton. Xenomorphs are capable of adapting themselves to cold environments with little discomfort. The alien queen is extremely vulnerable to fire, for unknown reasons.

Xenomorphs are shown to be both invulnerable and vulnerable to small firearms, ranging from 9mm to 5.56mm. This is seen in one scene in Aliens where Vasquez and Lt. Gorman find themselves trapped in the ventilation between advancing aliens coming from both directions, Gorman fires with his pistol on one of them with no effect at all, instead it looks like they bounce off (presumably from the head carapace). In an earlier scene, Vasquez holds an alien at gun point and fires her pistol at the head, which apparently goes through and the alien rattles away, obviously damaged. High-caliber firearms, with the explosive rounds of a M41A pulse rifle may cause the creatures to detonate. High-energy beam weapons, such as the plasma weapons used by Yautja, appear to be the best way to kill the Xenomorphs.

They are not invulnerable to melee weapons, although such equipment is likely to be destroyed upon contact with their acidic blood, with a few notable exceptions.

Cocooning
Cocooning is a trait used by Xenomorphs to reproduce quickly. A human, or any other creature, if dragged away, would have a cocoon of saliva and resin placed over them (their face left free), usually in an area where the Queen has produced a large quantity of eggs, so that the Facehuggers are able to latch onto them easily and deposit a Xenomorph larva in their chest. In a cut scene in Alien (included in the Special Edition), Ripley found two of her cocooned colleagues actually being turned into Xenomorph eggs. This suggests that Xenomorphs can convert humans into eggs when there is no queen, although this may have been scrapped as many other features of the Xenomorphs were in later movies (such as the transparent skulls and empty eye sockets). In the book, it's described how the drones are able to implant a fetus in up to four hosts, thereby creating enough drones and warriors to start a hive off.

Another feature of cocooning as seen in the game Aliens Vs. Predator 3 is that the Xenomorph Number Six, after entering and cocooning its victims, can change its appearance, eventually becoming a queen itself. This rapid genetic regeneration is also another evolutionary trait of the Xenomorph to ensure its very survival as a species. In Alien vs. Predators: Extinction the praetorian acted as a princess of the hive who then became the queen. This stage is seen with Number Six. If this is so, then Hudson is correct with them being like bees. The Xenomorph Queen can lay specialized eggs that are bigger (like queen egg cells in bees which are bigger than drone cells) that upon hatching can carry on the species through making other hives after the new female cocoons into a queen, as seen with Number Six and the aliens of Aliens vs. Predators: Extinction. If this is the case though, that a drone can become a queen when no queen is present, then cocooning is also a method of survival for the Xenomorph species.

Variations
The Xenomorph has been portrayed in noticeably different ways throughout the film series. Much of this was due to the continuing advancements made in the field of special effects, technology, and techniques used to bring it to life.

It has also been shown, both on-screen and in the games based on the Alien series, that the Xenomorph's form is affected by its host, so that different species of hosts will create different varieties of Xenomorph. An example of this is seen in Alien³, when a dog (an ox in the extended edition) spawns a Xenomorph with a distinct canine body structure, quadrupedal movement, and greatly enhanced speed.

Upper Body
In the original Alien film, and in the sequel Aliens, Xenomorphs are depicted as tall, slender creatures with a roughly human biomechanical design. The chest area appears to be rib cage-like in design.

In later films, such as Alien: Resurrection and Alien vs. Predator, they are depicted as being shorter and bulkier organisms, as well as being more quadrupedal, portrayed by either stuntmen in suits for close-ups, or computer-generated imagery for their full form.

In Aliens, Xenomorphs are depicted as having ridges along their cranium, while in all other films they have smooth cowls covering their skulls. It was speculated in the Aliens bonus DVD that this was due to these Xenomorphs having had more time to mature, compared to the Xenomorphs in the other films.

In the comics and various video games (Aliens vs. Predator: Extinction, Alien vs. Predator (SNES), Aliens vs. Predator 2), this was depicted as one of the visual differences between the Xenomorph Drone and Warrior castes. The Drones (in this instance used to indicate a "worker" caste, instead of mating-capable males) have the smooth cowl while the Warriors display the ridges. The Drones and Warriors have different tasks: warriors defend the hive, and drones hunt potential hosts, while some live only to protect the queen, such as hive warriors or praetorians.

In Alien and Alien³, the Xenomorph has six fingers, with the index and middle fingers conjoined into one digit, ring and little fingers also conjoined, and thumbs on both sides. In Aliens, the Xenomorphs are shown with five fingers, with the second thumb missing. In Alien: Resurrection and Alien vs. Predator, the aliens have four fingers, though the middle two appear to be a fusion of two digits. No explanation is given in the films for the fluctuating number of digits (this being similar to Yoda's digit numbers in the Star Wars series changing each film).

Lower Body
In Alien and Aliens, the Xenomorphs were plantigrade. In Alien³, the Xenomorph is digitigrade, owing to its quadrupedal host organism (a dog in the theatrical cut, an ox in the extended version).

Alien: Resurrection saw a dramatic redesign from previously-encountered human-spawned breeds: the Xenomorphs that emerged from human hosts in this film walked on digitigrade hind legs in a manner that does not conform with the appearance of previously-seen members of the breed. This is complicated by the fact that the Xenomorphs of the film were spawned by a Queen grown from cells derived from a clone of Ripley, making their genetic heritage muddled at best.

In Alien vs. Predator, the Xenomorphs return to their roots as plantigrade organisms, though their legs are sleeker and more skeletal in appearance, due to the use of computer visuals and hydraulic puppetry, rather than costumes.

Originally, the Xenomorph's tail was roughly the length of the rest of its body, with a small, almost surgical stinger-like barb on the end; but from Alien³ onwards, the tail has extended in length and features a large, knife-like blade at the tip.

In Alien: Resurrection and Alien vs. Predator, the tails have also supported a ridge of spikes right before the blade. This was introduced in Alien: Resurrection to help them swim convincingly, and was left intact in Alien vs. Predator. The tail itself is shown to be of incredible length during Alien vs Predator, measuring almost its full body length.

The original shooting script for Aliens featured a scene in which Lieutenant Gorman was "stung" by a Xenomorph's stinger. He was not killed, merely stunned, and the barb remained lodged in his shoulder, having torn loose from the Xenomorph's tail much like a bee stinger. The novelization also included this scene, though the final cut of the movie does not. This attribute is used later on in various games.

The Head
The most notable feature of the Xenomorph species is the elongated, banana-shaped head. This aspect is both disturbing and terrifying, combining visual elements that are usually blocked by our conscious mind with others resembling vaguely human features.

Though it appears to lack any sort of optical vision components (such as eyes), it is believed that it "sees" by sensing pheromones. This can neither be confirmed nor denied, but is the most widely-accepted hypothesis due to the fact that a Drone will not kill a host body already infected (as seen in the movie Alien 3 and the book Aliens: DNA War). There has been one detail on an alien's head that may be an eye being so small and the same black as the skin. It is hard to see if these are eyes or an implant on the skin (you can see them on the front cover of the Aliens vs Predator 2010 video game and very rarely as a predator in alien vision).

Other proposed conjectures include that the eyes are similar to a desert-burrowing lizard's eyes; meaning that they lay under a thin layer of skin, yet are deep enough that they are not visible from the outside. Another idea is that their whole head is one big insect-like eye, but there is no proof of this. In Alien 3, the fish-eye lens represented the Runner's sight, but it is unknown if this is really how it sees.

Despite this, the Xenomorphs could theoretically see. This is seemingly lent support by early models and toys of the Warrior Xenomorph while Alien was still the only existing movie in the timeline. These showed an actual, physical, humanoid skull built under the smooth carapace, with the smooth top working like a one-way visor.

Dorsal Spines
Along the back of most Xenomorphs' bodies are a series of appendages that appear to have no function. It's still in debate what they are used for, with theories ranging from sensory organs to communication organs, to breathing apparatus, to defensive mechanisms, to structures that dissipate heat, to storing substances their bodies need, or to provide structures to carry around the young, underdeveloped chestbusters, the eggs or the facehuggers. None of these ideas has been confirmed or denied by official sources so far.

One theory is that the four back appendages are used as a type of radar that the alien uses to see through extreme environments or detect movements. However in the extended universe it is said that the dorsal spines are used to secrete and possibly store the resin used to build their hives.

It is interesting to note that, in the Alien vs. Predator Volume 1 comic, Xenomorph Drones bury themselves in the ground, yet their spines are still shown just breaking the surface, implying some use when connected to the environment. This may be either biological (breathing, heat exchange, etc.) or sensory (detecting the vibrations of marines walking on the ground above).

Other hypotheses include:
 * Perhaps the spines separate the dominant Xenomorphs from the weaker ones, for example, the smaller the spines, the weaker the Xenomorph, the bigger, the stronger.
 * The creatures might come from a mainly amphibious planet, and perhaps the spines are a mechanism to swim underwater faster to catch prey. They do not seem all that hydrodynamic, but it's possible to suppose the spines might be used for stability, like the dorsal fins of a dolphin (not that their shape seems very much efficient at that either).
 * The spines might be spiracles that take in and filter the air around them, while waste is expelled through the mouth. It is widely accepted that they do not have lungs, so this is possible, and yet implausible because Runners, or Dog Aliens, don't have them. But it may be that the Runners have lungs.
 * The spines might be used as a defense mechanism, to repel enemies that try to sneak up on them from behind or above.
 * They might also be traits that the alien acquires by chance when developing inside the facehugger, putting legs of the facehugger on the back. The chestbuster does not develop directly from the facehugger (instead, the facehugger plants the egg/larva that will become the chestbuster inside a host), but it's not impossible that it acquires genetic traits from it.
 * They might be used to blend in with the hive web when under attack.
 * The spines may be for Chestbursters to latch onto, allowing the adult to bring them to a safer location.
 * If the alien uses echolocation to see its prey, these may be what they feel the vibrations from.
 * They may be used to carry unhatched eggs to a new location when the Queen dies giving them the ability to still climb walls while on the move.
 * They might be used to create the hive somehow.
 * The spines may be used to distribute pheromones - to distinguish one from another, and from other hives.
 * The spines may act much like the Carrier Xenomorph form seen in Alien Vs. Predator: Extinction, allowing a Facehugger to hook their spider-like legs until a victim is near. The Facehugger will let go of its crude form of transportation, find the victim, and proceed with the process of laying eggs.

Secondary Jaws
The secondary jaws, also known as the inner mouth, double jaw, or maw, is another disturbing feature of the species. It's assumed that it's powered by extremely strong muscles inside the creature's head, that enable it to be quickly launched and retracted, similar to the tongue of a chameleon.

It is the Xenomorph's close-combat weapon, as opposed to its long-range tail. The Xenomorph lashes it out of its mouth, and it destroys anything it hits, able to put a hole in a thickened Yautja skull and still have enough power to plunge another hole through the back as it escapes.

The secondary jaw is powered by muscles as seen in the first Alien vs. Predator movie when the predator rubs a part of the interior (resembling the gills of a crab) a feature that can be seen from the external midsection on each side of the head, ovular in shape. Muscles rub along here, letting the mouth be pushed out like a piston through the target the Xenomorph has acquired.

The Predalien has the ability to use its inner mouth to implant Chestburster embryos into a host's throat, like a Facehugger.

Yautja take them as souvenirs of their kill.

The Tail
It is accepted that the tail is an offensive weapon and little more. Though the creature can and will run on all four appendages, it often stands on its two hind legs so it can reach for its prey with its arms. While standing, it has also been seen to impale its victim with its tail, even to the point where it can raise a creature heavier than itself to eye level (if it had eyes), possibly in preparation for a face-bite using the secondary jaws. This would clearly be impossible if the tail were needed for balance, as the already-shifting weight would be too awkward, causing the monster to fall.

As a weapon, the tail is extremely efficient, as it is long enough to attack from great distance and can be used in a variety of ways; for example, in Alien vs. Predator, a Xenomorph used its tail to knock the legs out from under a Yautja before trying to stab it.

The tail can be used in stealth attacks, since it is flexible enough to attack from a variety of positions, and is hard to notice if it is behind the enemy, or if the area is rather dark. Even when the sharp point is cut off, the tail is still deadly, as the Xenomorph can use the tail to spray its acid blood from the severed tip over a wide area. Xenomorphs are also capable of using their tails as blunt-force weapons, clubbing their enemies with the vicious tail spikes.

Acidic Blood
The body fluid of all Xenomorphs appears to be a highly concentrated acid. It is mistakenly called "acid blood", but when it is compared to the blood of every other known life form, it doesn't seem to carry any nutrients through the body.

It is also highly unlikely that it carries oxygen for a few reasons: the first being that the Xenomorph has been known to survive in vacuums such as space (for short periods of time, at least). The second being that, in the Director's Cut of Aliens, Bishop mentions that the fluid loses its acidity shortly after the creature's death due to oxidation. This is plausibly unimportant as oxidation is a term in chemistry used for the loss of electrons rather than a reaction with oxygen. All acids are oxidized no matter what they react with, therefore this has no bearing on blood carrying oxygen.

The "acidic blood" is able to spread through almost everything except the Xenomorph's body, as shown in Alien vs. Predator. It apparently knows how corrosive its "blood" is, as one flicked it at a charging Predator, and it has been used to escape cages (by killing a "brother"). Some specially-treated Predator weapons can resist this acid, although how long exactly is unknown. "On the pH scale, it would probably rank at a devastating negative four or lower", a previous author claimed.

This is theoretically possible, despite the fact that pH is the minus logarithm of the free H-ions' concentration. To put this this into perspective water has a neutral pH of 7 and our stomach acid has a pH of 1, which would lie at the very bottom of the 1 to 14 scale. Attempts to discredit such a claim are challenged indirectly by Chemists who have found that mixing antimony pentafluoride with hydrofluroic acid can produce a solution with a pH of -31.This therefore would render it 100,000 billion, billion, billion times more potent than stomach acid. It should be noted however that this acid is basically formed by mixing two incredibly strong acids together in the first place, which interact to become even more acidic. Though former theories have in fact stated that two potent chemicals (with perhaps some biological catalyst) may mix to create the acidic blood seen. Perhaps the strongest 'solo' acid would be the boron-based carborane with a pH of -18. Yet due to the structure of Carborane, the H+ ions, which create the phenomenon known as acidity, are shared so efficiently between molecules that it would have no corrosive effects at all. Hence despite challenges It would appear even that which is known on earth certainly leaves scope for the incredible properties of the aliens acidic blood. It may also go without saying that different environments in other worlds could evolve "acid blood" that could also carry whatever nutrients that the Xenomorph needs.

It has been often theorized that the Xenomorph "blood" may serve no function other than defense, being a liquid stored under the creature's skin that causes great harm to whoever succeeds in harming the Xenomorph. It remains possible, though, that the acid serves some function in the alien's metabolism, perhaps even as an actual blood system analogue. One possibility is that it may be an extreme analogue of our stomach acid, and likewise serves to digest food. In that case, the Xenomorph digestive system would be spread like a circulatory system throughout its body: the acid is so powerful that anything it consumes is immediately dissolved and the remains distributed over its body, with the extremely resistant walls of the "acid vessels" somehow allowing nutrients to be absorbed.

Genetic Adaptation
Due to horizontal gene transfer during the gestation period, the Xenomorph also takes on some of the basic physical attributes of the host from which it was born, allowing the individual alien to adapt to the host's environment.

Through experiments, it was discovered that the embryo attaches itself to a major artery of its host, to get nutrients as it grows. It is presumed that the DNA from the blood it acquires infuses itself with the embryo's DNA to make up for its lack of genetic structure.

It may take on a thicker, bipedal form with a darker exoskeleton if it comes from something that stands on two legs, such as a human. In the movie Alien 3, it came from a four-legged animal, and thus took on a more slender, sleeker appearance where it too moves as a quadruped, with a more reddish/orange exoskeleton. It may also take on some facial features of its host (such as mandibles when implanted in a Yautja). Predaliens (Yautja-Xenomorph hybrid offspring) have a yellow-green partial skin covering their exoskeletons.

Furthermore, it is conceivable that it derives some level of intellect from its host. The quadrupedal variant in Alien 3 seemed easier to trick than previous bipedal versions and the Predalien appeared to have a better understanding of weapons being used against it.

Egg
The Xenomorph is born in egg form, laid in place by the hive's Queen, through the use of a detachable ovipositor. The egg is thick and gelatinous, usually with several membranes attached to the outer layers. It can take nutrients from the ground to support the creature inside. The egg has vein-like structures running through it, hinting that it is actually a living creature (sometimes known as an Ovomorph).

The Xenomorph hatchling - called a Facehugger - is protected by a thick fleshy layer at the top of the egg, where the "shell" peels open when the Facehugger senses creatures nearby. The sense that detects nearby, suitable organisms has to be very specific in what it chooses to respond to. Perhaps this is why it takes so long to open: various sensory organs study the approaching organism.

In the first movie, it appeared they could sense when another creature crossed into some form of mist covering their nursery area. It's a widely accepted hypothesis that this was a containment field to keep the eggs dormant (this same mist has been used as such in Aliens Vs. Predator 2 video game), put in place by a Space Jockey. When this mist was broken, the Facehuggers appeared to be alerted, and went on the search for the host.

Facehugger
After the egg opens, the Facehugger emerges and seeks out a nearby life form, latching onto its face, wrapping its tail around its airway, and implanting the Xenomorph embryo(s) by sticking a tube down the host's throat. It is seen sometimes when a facehugger is contained in water or lunging at a soon to be host.

It will coil its tail tighter around the airway to prevent premature removal, and an attempt to sever the fingers found that Facehuggers possess the same acidic blood as a Xenomorph. The Facehugger supplies oxygen to keep the host alive during the impregnating process. Once the embryo(s) has been implanted, the Facehugger falls off, curls up, and dies.

Hosts usually fall into a short coma and wake up ravenously hungry. This can be explained as the embryo needing an immediate source of nutrients needed for its progression into the next stage of its life. Hours might pass between the host waking and their time of death. Predators live the longest as a result of an extended incubation period. The reason for this is likely due to the genetic complexity of the host. The incubating chestburster requires more time for mitosis and cytokinesis due to the morphology of the Predalien.

However, in some cases, the hosts will awake not gradually but suddenly - just in time to be conscious for the Chestburster's push out into the world. Their state of normalcy will last only seconds or minutes.

It was also proven - in the movie Alien - that when a host awakens, there is some sort of a memory lapse before the Facehugger actually attaches itself to the host; they don't remember anything that has happened to them. This short term memory loss is most likely a result of the temporary oxygen deprivation caused from the Facehugger's tail, as it asphyxiates the host.

Chestburster
After attaching itself to a vein and gestating in the chest cavity, Xenomorph infants - called Chestbursters - push and chew through the host's chest, causing severe pain, blood loss, shock and death. The host's rib cage is cracked open, bones bent outward. No one has survived this process. However, Ripley, in the film Alien: Resurrection, was cryogenically frozen after she was cloned, and then had it surgically removed. It was also revealed in the 2010 Aliens v.s. Predator game that high amounts of stress can actually make the Chestburster more frantic. This causes it to push and chew through the chest at a faster pace, therefore emerging slightly earlier than is normal.

At this stage, the Xenomorph is small, limbless, and very vulnerable. Among other possibilities, its neck can be snapped. It needs to get to a safe area so it can reach its next life stage. It moves with its long tail, and can coil up and jump. It cannot climb on walls or ceilings, but despite these drawbacks it can traverse an area with astonishing speed.

In Alien vs Predator: Extinction, they turn into a cocoon (though this could be a subspecies). The common assumption is that the alien molts, grows legs, and develops much like a tadpole turning into a frog. Another common theory (the most accepted one) is that Chestbursters shed their skin upon growing, as shown when Brett picks up a Chestburster skin he found on the floor in an area the Chestburster took shelter in.

At very rare times the chestburster emerges after the host has already died, as seen in Alien 3 extended version. Why the ox died after it was implanted is unknown; It is possible that due to the size of the ox's neck, the facehugger cut off too much air. Strangely, in this circumstance, the chestburster lived. This was also seen in Alien vs Predator: after the Queen killed the Predator, the chestburster survived.

Adult
Little is known about the transformation that occurs between the Chestburster and Adult phases.

The Xenomorph appears to moult before reaching maturity. Maturity is reached in a few hours, and involves a dozen-fold increase in mass, which would presumably require some form of nourishment. In the novelization of the movie Alien, Ripley comes across a food locker that had been raided, apparently by the Xenomorph to get food. Whether or not this was nourishment to grow was not specified. In the video game Alien Vs. Predator 2, one of the Alien missions requires the player to find a source of food in order for the Chestburster to advance to its Drone phase. At the end of the level, a short cut-scene indicates that the Chestburster consumes a house cat in a carrier. The scene does not show what happens during the change in life cycle.

In Alien, Brett found a shed skin resembling a Chestburster in the area of the Nostromo it had taken shelter in, implying that it sheds its skin before moving onto its next form. This has also been shown in the expanded universe.

When the first Drone encountered on record was born on the Nostromo, it had a smooth, long head, and an upright-standing body. It had no eyes, a tail, and strange spines protruding from its back.

However, many years later on LV-426, the same warriors were found, but they looked slightly different. The head was no longer smooth, but ridged. One theory is that the smooth-headed Xenomorph was a Drone, and the ridge-headed Xenomorphs were Warriors (The Xenomorph in the first movie built a nest, which only Drones can do since it is their job in the colony).

The most widely accepted idea, however, is age - the Xenomorphs found on LV-426 were more than a few days old, where all of the others encountered were less than a day old. On the other hand, the Alien vs Predator game has showed that the Drones are black (like in the movies), but the Warriors are a very dark blue, the color difference indicating that age is not a factor.

Another variation is the Dog or Runner Xenomorph, called so because of its dog-like stance (standing and moving on all fours) and higher speed. Its host was a dog, rather than a human.

Another variation was encountered on an expedition to Antarctica. These warriors were less humanoid and sometimes walked on all fours. Their tails were longer, with a large, curved barb at the end. The head was smooth, but much shorter than normal. It is important to note, however, that this particular Xenomorph hive was grown by the Predators, who introduced a hormone with unknown effects into the Queen's system.

As the Xenomorph has been shown to be an extremely adaptable organism, which lays hundreds of eggs, develops quickly and is highly efficient at altering its own genome to better fit the environment (absorbing traits from its hosts), it's very likely that some phenotype variations are simply the result of genetic variety between different populations, which evolve very fast.

Xenomorph Caste System
The Xenomorphs have a caste system similar to those of bees and ants. A major difference is that their caste system is far more complex.

Drone
The basis of any Xenomorph hive; the Drones are very much like worker bees or ants. Their main tasks are to keep the hive in good condition and protect it from attackers.

The Queen secretes hormones that keep them sterile, and when she dies, the Drones go into a rage, fighting with each other until they find the strongest among them. Which the winner female is usually already molted into a Preatorian. That Female will then evolve into a Queen and immediately begin to lay eggs. She will also choose whether to keep the Praetorian consorts of her predecessor, or choose a new pair of Warriors for the role.

They average in size at about 6–7 feet tall standing on hind legs, and about 14–15 feet long, tail included. They have the basic body build of the Warrior, except that their dome is completely smooth, where as the Warrior's dome is more ridged. Drones are smaller and somewhat more delicate than the Warriors, not considered quite as dangerous by Yautja standards, but striking fear into the heart of any human.

Like all of their brethren, they have acidic body fluids, but only the Drone can spit this secretion onto prey over a short distance.

Drones have an organ in their body that releases a sticky fluid that they can spit out of their mouth onto any surface. It can be used to harvest hosts for the Queen's Facehuggers.

Warrior
Warrior Xenomorphs - commonly known as simply 'Warriors' - are the soldiers and protectors of the hive. They are all children of the hive's Queen, bigger and stronger than the Drones. The Drones are less deadly, and somewhat less intimidating.

Warriors average in size at about 8 feet tall standing on hind legs, and about 14–16 feet long, tail included. In most cases, the Warrior looks the same as a Drone, except that the dome on top of the head is completely gone.

They evolve into Praetorians, which serve as the Queen's consorts. Typically, there are four of them at any given time. However, when the Queen wishes to watch them battle for her attentions or increase protection in preparation for an attack on the hive, she will release pheromones which cause four additional Warriors to evolve into Praetorians.

Praetorian (Royal Guard)
Praetorians are bigger and stronger than the Warriors and are seen to have a developing crown. Their primary role within the hive is to guard key locations of importance or interest, and, if nothing else, to protect the Queen herself. Most Praetorians can be found very close to the Queen's chambers. The physical features of the Praetorian are pristine, and very similar to the Queen's: it has a crown-like head crest, large size, and great strength.

In Alien vs. Predator: Extinction, the Praetorian is said to be created by special eggs and Facehuggers. However, it is widely accepted that Praetorians evolve from the Warrior caste. The Queen selects a Warrior to become a Praetorian, and the Warrior is then attacked by fellow Xenomorphs from the same hive and banished. The Warrior must survive on its own for a long period of time. It grows and sheds, becoming approximately fifteen feet tall and gaining its head crest. It returns to the hive as a worthy guard to the Queen.

In the 2010 Aliens v.s. Predator, the main Xenomorph, "Number 6", Molts/transforms in to a Praetorian then into a Queen after the first queen and hive was killed by an explosion.

Some varying reports of Praetorians with massive shield-like growths on their forearms have been seen.

Queen
The Queen is the largest and most intelligent Xenomorph and can grow incredibly large, up to 100 feet tall if given time. She lays many eggs that hatch to become the first stage of Xenomorph. It is unclear whether the Queen engages in combat outside of protecting her hive. The Queen is usually nestled deep within the hive, protected by Praetorian guards.

She has been known to display some logic-based intelligence, primarily in using simple human technology such as lifts. She is the most dangerous of the hive, displaying extreme aggression and using her intelligence to devastating effect.

When attached to her egg sac (ovipositor), she is immobile and vulnerable to attack. When she removes herself from it, however, she can move surprisingly fast. She attacks with her four clawed arms, as well as her long, bladed tail. She has also been shown to use her main and inner jaws in combat.

Interestingly enough, it seems that the size of Queens vary. In Aliens, the Queen was shown to be maybe only twice the size of a Drone. In AVP, she is shown to be considerably larger - three or four times the size of a Drone. This could be explained in a similar fashion to an older Drone's ridged head. The Queen in AVP was very old; although kept in a dormant state for long periods of time, she could have been alive for several millennia, or she could have been an Empress. Queens usually stand around 15 to 20 feet tall.

When there are no Queens within a certain hive mind range, a Drone will be produced in chestburster stage. After making a nest, the Drone will go through a hormonal change and become a Queen, producing an egg sac and eggs. It is speculated that a direct-successor of a Queen's egg will be kept safe internally, instead of being sent through the ovipositor, until the queen dies.

Empress
In the video game Aliens vs. Predator 2, there is mention of a Xenomorph Empress. Although very similar to a regular Queen, the Empress's main duty is to establish order on a planet with multiple hives. The only known differences between the two are their size (the Empress is substantially taller than a Queen, standing anywhere from 20 to 25 ft. tall), and the crown (an Empress' crown has five points instead of the traditional three). It is possible that an Empress is simply the oldest queen on a planet, and the physical differences are the result of aging.

Dr. Eisenberg might have merely used the term "Empress" to refer to the Queen. However, the Empress is bigger than the average Queen seen in the Predator and Marine missions.

It is also mentioned that after a hive grows to around 2,000 members, the younger Queens set out to establish new hives, and that if the Empress was to die, it would cause the younger Queens to fight over the new hive ruler. It's most likely that an "Empress" Xenomorph is merely a Queen that has dominated the other Queens in a hive. This would make the title "Empress" just that: a title. But in Aliens vs. Predator 2010, a very old Queen has a white skin color leading to the possibility that this Queen could have been an Empress.

Predalien
Like normal Xenomorphs, a Predalien gains traits from its host. But from the Chestburster stage, it is noticeably different from other Xenomorphs. It is a somewhat dark shade of green with external mandibles, due to its Predator (Yautja) heritage.

They lack speed, and their acid blood is not as deadly as a normal Xenomorph's. It will glow like a Predator's. If a Predalien is part of a hive that has no Queen, it may serve as Queen due to its reproductive capabilities - its mandibles hold the host's head in place while its inner mouth implants one or more Xenomorph embryos into the host's throat, similar to the function of a Facehugger. Up to four Xenomorph embryos may be planted in a host, if the Predalien needs to build up numbers quickly. It also possesses higher intelligence than normal Xenomorphs.

Design
Though maintaining the basic design shown in the Aliens vs. Predator video games, numerous artistic liberties were taken in its film portrayal. Or rather, would have been taken. One abandoned concept was to have the Predalien reproduce by injecting victims with "DNA acid" through its tail, causing the victim to swell and explode.

Another is the Predalien's inherited tendency to skin victims. This was probably dropped because skinning victims isn't something that comes instinctively to Predators, but rather a learned behavior.

There is also many different kinds of xenomorphs not listed here. For more information, check the "See Also" Section below.