Xenomorph


 * Alien redirects here. For extraterrestrial species in general, please go to Category:Alien Species

The Xenomorph (Latin, Internecivus raptus, "murderous, deadly" and "seized") 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 and will kill any non-xenomorph life form on sight, regardless of age or species, with the exception of the individual that has been implanted with a future Queen, which they will not touch for fear of harming said developping leader. 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 the species is generally twice as large 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 can neither be confirmed nor denied. 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. (This theory has been twisted slightly by Prometheus, as they created an artificial subspecies, the "Deacon" or proto-alien. (Deacon (Prometheus))

Another theory is that they were actually the oldest and most technologically advanced of all races, but a sort of plague wiped out all the adults, leaving the offspring to devolve into a bestial state. The Mala'kak (Space Jockeys) may have been a close ally, possibly explaining why Xenomorph eggs were on their ships to LV-223 and LV-426 (transportation away from the plague).

Other ideas include that they were created by another race, perhaps the Yautja (Predator) as an "ultimate prey". Xenomorphs are specifically said to not be indigenous to LV-426, a relatively small and unknown planet, where a large body of their eggs were found in the hold of a derelict Mala'ka/Space Jockey ship. No one knows if they were using the eggs as research or as a bio-weapon. Some say that billions of years ago, the advanced race (probably the "Space Jockeys") controlled much of the universe with the Xenomorphs, but were wiped out by them.

Biology
Little is known about their biology, but it is known that they eat with their "tongue mouths," not their real mouths. This mouth is like the pharyngeal jaw of a moray eel.

Intelligence
Xenomorphs appear to possess an intelligence roughly similar to that of a typical primate. 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 had more time to learn how to manipulate machinery, if only at the most basic level.

Xenomorphs seem to have some resemblence to their host, though not much. When a predator got infected the parasite became a predamorph or predalien. The mouth of the predamorph had the extra mouth peices like predators.

Xenomorphs have demonstrated little emotion, though they are not completely devoid of fear, especially for their eggs. 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 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 attack tongue.

In the rare event that a xenomorph finds itself under attack or otherwise vulnerable, it displays 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 dons 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 similiar to Chitins. He also says the creature replaces its surface cells with polarised 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 claws capable of slashing through some types of metal (as seen in ALIEN books, such as Book 2: Nightmare Asylum). 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 sting prey.

It stands around 7 feet tall, averaging in at anywhere between 140 kg and 180kg. Its mouth is filled with sharp teeth, but its main killing device is the inner mouth that shoots out to penetrate the victim's brain.

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. 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 biomechanical appearance.

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 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 dont kill Xenos.

Adult xenomorphs are quick and agile, and can run 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 or a special gland somewhere in their throats. 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 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 underneith to be exposed. In Alien³, a fisheye 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 creatures 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 detect their prey using pheromones.

Another theory is that the eyes of a Xenomorph is 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.)

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.

The only xenomorphs not vulnerable to fire are the Predaliens. In the game Alien vs. Predator: Extinction, they secrete a layer over their body, and with that layer cannot be caught on fire, though they are visibly pained by 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.

Xenomorphs are shown to be both invulnerable & vurnerable to small firearms, ranging from 9mm to 5.56mm

As seen in one scene in Aliens where Vasquez & Lt. Gorman finds themselves trapped in the ventilations 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 hold an alien at gun point and fires her pistol at the head, which apparently goes through and the alien rattles away, obviously damaged. and high-calibre firearms, with the explosive rounds of a M41A pulse rifle causing the creatures to detonate.

High-energy beam weapons, such as plasma weapons, appear to be the best way to kill or destroy the xenomorphs.

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

Cocooning
Cocooning is a trait used by xenomorphs to reproduce quickly. An injured (but alive) human, or any other creature, is dragged away, and a cocoon of saliva and resin is 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 fetus 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 transparent xenomorph skulls and empty eyesockets.) In the book it describes how the drones are able to implant a fetus in up to 4 hosts there by creating enough to start a hive off

Another feature of cocooning as seen in Aliens Vs. Predator the game 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. We saw this stage with Number Six. If this is so then Hudson is correct with them being like bees. The Queen Alien can lay specailized eggs that are bigger (like queen egg cells in bees which are bigger than drone cells) that when they hatch 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 and quadrupedal movement.

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.

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, some might argue that the drone and warrior are the same but they are wrong. They both have different appearences and roles, as explained below.

The Drones and Warriors have different tasks: warriors defend the hive, and drones hunt potential hosts, while some live only to protect the queen like 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.(Similar to Yoda's digit numbers in the "Star Wars" series changing each film.)

The chest area appears to be rib cage-like in design.

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 alien vs predator game 2010 and very rarely as a predator in alien vison).

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 Alien'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 are a series of appendages that appear to have no function. It's still in debate what they are used for.

One theory is that the four back appendeges are used as a type of radar that the alien uses to see through extreme enviornments or detect movements.

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 seperate the dominant Xenos from the weaker ones, for example, the smaller the spines, the weaker the Xenomorph, the bigger, the stronger.
 * The creatures come from a mainly amphibious planet, and perhaps the spines are a mechanism to swim underwater faster to catch prey.
 * The spines are spiracles that take in and filter the air around it, while waste is expelled through the mouth. It is widely accepted that they do not have lungs, so this is possible, and yet impossible because Runners, or Dog Aliens, don't have them. But it may be that the Runners have lungs.
 * The spines are used as a defense mechanism, to repel enemies that try to sneak up on it.
 * They are traits that the alien gained by mistake when inside the facehugger puting legs of the facehugger on the back.
 * They are used to blend in with the hive web when under attack.
 * The spines are 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 are used to carry unhatched eggs to a new locaton when the Queen dies giving them the ability to still climb walls while on the move.
 * They are used to create the hive.
 * The spines are used to distribute pheromones - to distinguish one from another, and from other hives.
 * The spines act much like the Carrier 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.

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 inertia to plunge another hole through the back as it escapes.

The secondary Jaw is powerd 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 has 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.

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 (plausibly unimportant as oxidation is a term in chemistry used for the loss of electrons rather than a reaction with oxygen. All acids are oxidised no matter what they react with, therefore this has no bearing on blood carrying oxygen.)

The "acidic blood" is able to eat through 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, however, is impossible, because the pH is the minus logarithm of the free H-ions' concentration, and thus can never be lower than -2.5 or so. Super acids with higher H-concentrations than this are merely theoretical, and cannot remain in this state because they immediately react and lose their unstable energy, by oxidation or neutralization. Another possibility is the presence of catalyst or enzymes in the blood which would increase the rate of reaction. However, we are limited in our knowledge of universal biology, as we simply base it off of what we know on Earth. Different environments in other worlds could evolve "acid blood" that could also carry whatever nutrients that the Xenomorph needs.

Genetic Adaptation
Due to Horizontal gene transfer during the gestation period, the Alien 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 Predator). 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 quadrapedal 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 Versus 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 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. Hours might pass between then 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.

The ravenous hunger can be explained as the embryo needing an immediate source of nutrients needed for its progression into the next stage of its life.

However, in some cases, they 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, and shock. 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 a area the Chestburster took it 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 dozenfold 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 it's Drone phase. At the end of the level, a short cutscene 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. Only Drones can build a nest - that is their job in the colony.)

The most widely accepted idea 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 it's dog-like stance (standing and moving on all fours) and higher speed. It's 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.

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. That Drone 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. 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 most references, a Praetorian is the last stage of a "Male" Xenomorph's life cycle.

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 it's 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", 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 sheild-like growths on their forearms have been seen.

Queen
The Queen is the largest and most intelligent Xenomorph in the hive. 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 internaly, 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's 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 then 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 it's 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.

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

Trivia

 * thumb|300px|right|Conker fights HeinrichBesides merging with the Predator franchise, the Xenomorph species has been featured in a myriad of other science-fiction and superhero sagas. It has been fought by several DC Comics characters, including Batman, Superman and the Green Lanterns. Xenomorphs also make frequent appearances in the humorous comic strip Brewster Rockit: Space Guy!. The Xenomorph Queen has also briefly appeared in an episode of the comedy series Family Guy.
 * Xenomorphs have also been featured or mentioned in a few of Star Wars comics and novels, and while their canonical status is highly debated, this information implies that the Xenomorphs at some point managed to travel to, and colonize, a planet in a far, far away galaxy.
 * The Xenomorph's method of reproduction is rather reminiscent of that employed by the Ixtl, a species featured in the novel The Voyage of the Space Beagle. The first Alien film has even been accused of plagiarism regarding the book, although the producers denied any possible influence or connection.
 * The Xenomorph's design has inspired the creation of the Metroid villain known as Ridley, which is a reference to Alien's director Ridley Scott. They also bear a resemblance to Space Pirates, the antagonist species in the Metroid game series.
 * In the Dragonball Z series, the first main villain Frieeza/Freeza has a third form transformation that resembles this species of alien life. (The signature long cranium.)
 * A Xenomorph named Heinrich serves as the final boss of the Nintendo 64 game Conker's Bad Fur Day.
 * Judging from the fact that virtually any Xenomorph can turn into a Praetorian and then a Queen, they are probably all female and reproduce asexually.
 * In Aliens, the xenomorph's seem to shake their head slightly before attacking their prey.