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" from Greek ξενος, xenos=strange and morphe=form) or the "Alien", is an extraterrestrial hive-based endoparasitoid species with a multi-staged life cycle, possibly originating from the planet Proteus (also known as "Xenomorph Prime").

One of the deadliest of all known alien species, these creatures require a host organism in order to reproduce. The appearance of the Xenomorph can vary depending on the host in which the embryo is implanted. The Human phenotype is generally around 7–8 feet (~ 2.13 - 2.43 meters) and roughly 136.0 to 181.4 kilograms in weight, with a long, muscular tail and large, curved, oblong head. The Queen of this species is generally twice as large (although some Queens have been shown to grow even larger, some surpassing 30 meters in height if given time) and possesses superior speed, strength and intelligence compared to her common offspring.

Biology
The physical form of a Xenomorph can vary widely depending on its life stage, hive, caste, age, and host. Its ability to incorporate genetic material from its host, coupled with the existence of numerous mutations and deviations which have originated naturally or artificially; makes the Xenomorph one of the most phenotypically diverse organisms in the galaxy.

A typical Xenomorph begins its life cycle as an egg-like creature (known as an ovomorph or simply egg) which, when detecting the presence of a potential host, opens up to release the second stage: a small arachnid-like life form (named the facehugger) which launches itself onto the host's face and impregnates it with an endoparasitoid larva. This parasite then consumes parts of the host from within and violently emerges from the host's chest cavity as a worm-like creature (grotesquely christened as the chestburster), which will quickly scurry away to find a safe place where it can molt and complete its metamorphosis into an adult Xenomorph, which happens, surprisingly enough, over the course of mere hours.

Although numerous varieties exist, adult Xenomorphs all tend to share some common traits: a vertebrate-like body, a protective dark exoskeleton, a long flexible tail, an elongated domed head and a toothed mouth hiding a secondary, extendable, proboscis-like inner maw. Little is known about their biology, but it is known that they eat with their inner or secondary mouths, not their outer mouths. This mouth is somewhat similar to the pharyngeal jaw of a moray eel. They also bleed an unknown and extremely corrosive fluid. This substance is dull yellow in color and often referred to as "blood", although whether it's used to transport nutrients through the body and/or serves principally as a defensive mechanism is not clear.

While the extremely corrosive fluids and tremendous resilience displayed by the creature might suggest an exotic 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. There is evidence that the genetic exchange is mutual (i.e. the parasite alters the host's DNA as well), given that Ellen Ripley's clones were created in an attempt to gain access to the Xenomorph Queen's genetic material, which wouldn't be possible otherwise.

Xenomorphs operate in hives made of a resin they produce, mixed up with their salivar fluids. The most commonly-observed castes are Drones, Warriors, Praetorians and Queens. The Queen can be generated in more than one way: from specialized royal facehuggers, or maturing from Praetorians, which in turn can mature from Drones or Warriors. 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; which is called parthenogenesis: a reproductive method practiced by many species on Earth and other planets. Alternatively, it's also possible that the aliens are either hermaphrodites or practice sequential hermaphroditism, i.e. being able to change sexes during life, as is the case with some species of fish and amphibians.

Intelligence
Most Xenomorphs appear to possess an intelligence level equivalent to that of a typical primate. It's well within their capabilities to learn relatively quickly, gathering information and making advantageous use of their environment. Although they do not demonstrate Human and Yautja-level intelligence as a species (such as abstract reasoning, introspection and technological advancement), their Queen appears to possess considerable acumen in her social behavior and manipulation of Human technology, such as cutting the power of the LV-426 colony and operating an elevator. While it may take considerable time for them to learn how to operate machinery, other Xenomorphs have displayed fast reasoning and the capacity to find creative, counter-intuitive solutions to problems, as exemplified by a group which, after noticing that they were being contained, intentionally murdered the weakest so that the acid of the Xenomorph's blood would eat through the floors.

As the adult Xenomorphs are known to inherit physical characteristics from their hosts (for example, when a Yautja was infected, the parasite 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 an 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, such as when they are in the presence of fire, which they are vulnerable to. They have also been shown to communicate pain through shrieks or screams when attacked.

It has been suggested that Xenomorph nests may operate under a hive mind, or a collective consciousness directed by the Queen, who can call back and give orders to her subjects, both telepathically and vocally with shrieks. In some cases, the Queen Mother Xenomorph communicates telepathically with potential hosts, inducing terrifying dreams and religious visions, which may even lead to the formation of cults with the Xenomorph as a god-like figure, not dissimilar to the cults of Cthulhu and other such entities. The most famous example of this phenomenon might have been Walter Golic, who received supposedly telepathic messages from a captured Xenomorph; which ultimately led him to release it and let it kill him. He called this Xenomorph "the Dragon", and viewed it as a god.

There is evidence that the Xenomorph hive mind also 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 can be seen following a Xenomorph escaping its cell. It had learned that the consequences of attacking the observatory glass was a jet of extreme cold (possibly liquid nitrogen), triggered by a large red button on the console. When a crewman entered the now-empty cell to study the hole through which the Xenomorphs escaped, one of them entered the control room and activated 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 in the afore-mentioned incident). They know that the darkness is their ally, and break lights to create cover.

Anatomy
A Xenomorph is equipped to be the perfect killing machine. They possess 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 silicon, which is a very inert, heat resistant, and flexible polymer. This allows these creatures great environmental resistance. However, they are vulnerable to sudden, massive changes in temperature. It's been demonstrated that they can be destroyed by immersion in molten lead followed by sudden cooling with fire sprinklers, causing their silicon shells to shatter.

Attached at the ends of both of their arms are a set of long fingers and shiny, black, extremely sharp claws capable of slashing through almost anything within their grasp. Their tail is long and flexible. Some, though not all, individuals also have ridges on top, and a serrated toxic tip at the end of the tail to lacerate enemies.

They stand at around 7 feet tall (~ 2.13 m), averaging in anywhere between 140 and 180 kg; although size may vary wildly depending on the host chosen, as seen for instance with the "Jock-Xenomorph": a gigantic creature spawned from a Mala'kak host.

A Xenomorph's outer mouth is filled with sharp teeth, but their main killing device is the inner jaw that shoots out to penetrate directly into 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, excluding the genetically-engineered Newborn Xenomorph. 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 Yautja armor.

The plasticity of the Xenomorph phenotype is one of the species' most notable features, and has resulted in physical traits that diverge from individual to individual and from hive to hive. As an example of how a host can influence the morphology and skills of the parasitoid, a Xenomorph spawned by a dog displays a distinctly canine body structure, digitigrate legs, quadrupedal movement, and greatly enhanced speed compared to those spawned by Human hosts, which are typically plantigrade and often bipedal.

Even among those spawned exclusively from Humans, different Xenomorph hives can produce highly divergent individuals. Some are taller and lankier, with an almost erect posture. Others are shorter and bulkier, and move more quadrupedally. While most have smooth craniums, others have ridges, which may be attributed to their age (i.e. those which have more time to develop may end up with ridges) and/or caste (ridges being associated with Warriors, and smooth features with Drones).

When it comes to their digits, there are at least three different patterns.
 * Some Xenomorphs have six fingers, with the index and middle fingers conjoined into one digit, ring and little fingers also conjoined, and thumbs on both sides.
 * Others are shown with five fingers, with the second thumb missing.
 * A third phenotype has only four fingers, though the middle two appear to be a fusion of two digits.

On at least one occasion, Human-spawned Xenomorphs were observed to have a digitigrade, as opposed to plantigrade posture. However, it should be noted that these were spawned by a Queen grown from cells derived from a clone of Ellen Ripley, making their genetic heritage muddled at best.

Some Xenomorphs have tails that are roughly the same length as the rest of their bodies, equipped with a small, almost surgical stinger-like barb on the end. This stinger can be, and has been, used to perforate victims, and may turn loose from the tail and remain lodged in the victim's body, not unlike a bee stinger. Conversely, other specimens have tails that are considerably longer, and armed with a large, knife-like blade at the tip and sometimes with ridges of spikes right before the blade, which may aid in swimming.

Head
The most notable feature of the Xenomorph species is the elongated, banana-shaped head. A potential explanation for this curious shape may be that the head may store muscles required to power the Xenomorph's inner mouth.

In terms of eyesight, the Xenomorph appears to be entirely eyeless. However, there are features on its head which could represent eyes, which are usually hard-to-see due to the fact that they're small and the same black color as the skin.

Other proposed conjectures include that the eyes might be 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. In that case, their eyes could be hidden 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 Earth fish, such as the Barreleyes, see through a transparent layer of skin; it is possible Xenomorph vision is similar. Another idea is that their whole head might represent one big insect-like eye.

Eyeless or not, it is widely assumed that a Xenomorph identifies enemies by sensing pheromones. This has never been explicitly confirmed nor denied, but a Drone will not kill a host that's already infected, so they must have some way of detecting the presence of the unborn parasite within.

Secondary jaws
The secondary jaws (also known as the inner mouth, double jaw, or maw) are another disturbing feature of the species. It's assumed that they're powered by extremely strong muscles inside the creature's elongated head, that enable it to be quickly launched and retracted, similar to the tongue of a chameleon. The albino Xenomorph Drone has a very long inner mouth that can secrete resin that's used to build the walls of their hives.

The inner mouth is typically the Xenomorph's close-combat weapon, as opposed to its long-range tail. The Xenomorph lashes it out of its outer mouth, and it destroys anything it hits, being able to put a hole in a thickened Yautja skull and still have enough momentum to plunge another hole through the back as it escapes. Yautja take them as souvenirs of their kills.

The secondary jaw is powered by muscles, as seen when a Yautja 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 Yautja-spawned Xenomorph (a.k.a. "Predalien") has the ability to use its inner mouth to implant chestburster embryos directly into a host's throat, like a facehugger. Since that particular Predalien was also on her way to developing into a Queen, it's possible that all future-Queens have this ability.

Dorsal spines
Along the back of most Xenomorphs' bodies are a series of appendages that appear to have no clearly-defined 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 chestbursters, the eggs or the facehuggers or as a counter weight to keep them from falling over when running. None of these ideas has been confirmed or denied so far.

One theory is that the four back appendages are used as a type of radar that the alien utilizes to see through extreme environments or detect movements. It is also 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 Xenomorph Drones have been observed to 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 sensorial (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 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 (a.k.a. dog-spawned Xenomorphs), 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 the legs of the facehugger on the back. The chestburster does not develop directly from the facehugger (instead, the facehugger plants the egg/larva that will become the chestburster 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, allowing them to still climb walls while on the move.
 * The spines may be used to distribute pheromones - to distinguish one from another, and from other hives.
 * The spines may allow 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.
 * The spines may also act as a counter balance, much like a cat's tail, so that the Xenomorph does not fall forward when running. Though in truth, a cat does not require a tail to balance properly while running, it more so helps to balance upon thin surfaces like fences and the backs of chairs, and since the Xenomorph already has a long tail, this idea has little credence.
 * They may be used for breathing, as it's possible that the inner jaws serve only offensive purposes. This also makes sense if one considers that they may have come from a swamp-like planet, where they would lay in wait for prey to wander by. This would also explain the 'dome' over their eyes (assuming they have eyes), which may have functioned similarly to a camel's second eyelids: allowing the alien to see, while preventing stuff from getting in its eyes.

The "Runner", a Xenomorph born from a dog or other quadrupedal host, is notable in that it doesn't have dorsal spines. This could be an adaptation to make its body more aerodynamic and thus more efficient for running.

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, possibly in preparation for a face-bite using the secondary jaws. This would clearly be impossible if the tail was 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, a Xenomorph has been observed to use 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 damaged 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.

Physical abilities
The adult Xenomorph is a living weapon, noted for their 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 their 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 their biomechanical-like appearance. The Xenomorphs will also use their pitch black carapace to their 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 earthly life form. An example of this is a Queen that survived a multi-ton exo-suit falling on her from over 30 feet. Xenomorphs are capable of surviving with limbs shot off, stabbings and even shots through the head (though total decapitation kills them).

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 respire in harsh atmospheres, and can survive in vacuum for indeterminate lengths of time. Their movements tend 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.

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 there is evidence that they can detect light and darkness as well. Xenomorphs may be able to find 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 locate their prey by its heart beats, even if the prey is immobile. This would explain how Xenomorphs always know where the Humans are. Furthermore, the Xenomorphs can also detect whether a potential host is already infected, likely by sensing pheromones.

Xenomorphs produce a thick, strong resin, which they use to build their hives and cocoon victims. Cocooning is a trait used by Xenomorphs to reproduce quickly. A Human, or any other creature, if dragged away, may 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 inside their chest. 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. The use of molotovs and flamethrowers constitutes an effective technique to drive them off. The only Xenomorph not vulnerable to fire is the Predalien, which can secrete a layer over its body that cannot be ignited, although they are still 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. However, the alien Queen is particularly vulnerable to fire, for unknown reasons.

Depending on the circumstances, Xenomorphs are shown to be both invulnerable and vulnerable to small firearms, ranging from 9mm to 5.56mm. This is seen when 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 the ammunition bounces off (presumably from the head carapace). In another occasion, however, 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, though pulse rifles appear to be the best way to repel large hordes of them since they can fire non-stop and are not prone to being destroyed like the plasma weapons.

Xenomorphs are not invulnerable to melee weapons, although such equipment is likely to be destroyed upon contact with their acidic blood.

Acidic blood
The body fluid of all Xenomorphs appears to be a highly concentrated acid. It is frequently referred to as "acid blood", although it isn't clear whether or not it's used to carry any nutrients through the body.

It is somewhat unlikely that this fluid 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 Bishop mentions that the fluid loses its acidity shortly after the creature's death due to oxidation. On the other hand, 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 dissolve through almost everything except the Xenomorph's body. The creature itself apparently knows how corrosive its "blood" is, as one flicked it at a charging Yautja, and it has been used to escape cages (by killing a "brother"). Some specially-treated Yautja weapons can resist this acid, although how long exactly is unknown.

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 stomach acid, and likewise serves to digest food. In that case, the Xenomorph's 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 more efficiently.

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. When the host is a four-legged animal, it may take on a more slender, sleeker appearance, where it too moves as a quadruped, and has a more reddish/orange-colored exoskeleton. It may also take on some facial features of its host (such as mandibles when implanted in a Yautja). Predaliens (Xenomorphs spawned from Yautja hosts) 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 seems easier to trick than bipedal ones, and the Predalien appears 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, showing 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 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. It seems to be capable of distinguishing synthetic life from organic life.

Sometimes Xenomorph Queens can lay eggs that contain up to four facehuggers.

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. This 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 an adult 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 for its progression into the next stage of its life. Hours might pass between the host waking and their time of death. Yautja 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 of the chestburster's push out into the world. Their state of normalcy will last only seconds or minutes.

It's also proven that when a host awakens, there is some sort of a memory lapse in which 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 by the facehugger's tail, as it asphyxiates the host.

Different types of facehuggers are known to exist, including some highly abnormal variants. The royal facehugger will give rise to Queens or to members of the Praetorian caste, the latter which may be promoted to Queens later on. However, there's also a poorly-known caste of giant facehuggers which are also known as queen facehuggers, but don't seem to be related to Queens directly. Unlike regular facehuggers, these giants don't seek to impregnate hosts, but merely protect the ovomorphs. They may use their legs and tail to immobilize potential hosts, allowing regular facehuggers to latch onto the victims' faces and impregnate them.

Another highly aberrant form is the Infectoid facehugger, which doesn't implant an embryo, but rather fuses itself to the host's body, transforming the host into an undead zombie-like being, linked to the aliens' hive mind.

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 was cryogenically frozen after she was cloned, and then had it surgically removed. It was also revealed 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 normal. On one occasion, the chestburster erupted out of the head of a host Human instead of the chest.

It is suggested that the removal of the chestburster would still lead to the death of the host due to the remaining alien tissue acting like a malignant tumor, in effect developing into a fatal cancerous growth.

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 some cases, 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, which has been observed in an ox. Why the ox died after it was implanted is unknown; though the most likely reason would be due to a combination of the stress of implantation coupled with the poor health of the animal. It is seen to be riddled with parasites and presumably spent much of its life in the harsh environment of Fiorina "Fury" 161. Strangely, in this circumstance, the chestburster lived. This was also seen after the Empress killed a Predator and the chestburster survived.

In some cases, morphological traits acquired from the host may already be visible in the chestburster stage, for example: a chestburster born from a Yautja will already display the characteristic mandibles of the race. Some types of chestbursters already have legs when they emerge, including the royal chestburster and the "Runner".

A variation of this stage is known as the bellyburster, which erupts from the host's abdomen, rather than chest. These seem to be formed when the host is a pregnant female, and the alien embryo develops inside her womb, presumably feeding on the fetus. A Predalien has been observed to impregnate several hosts with multiple embryos each, which later gave rise to bellybursters. Since multiple bellybursters can develop inside a single host's womb, they can be used as an efficient way for a young future-Queen to establish a new hive faster.

Adult
Little is known about the transformation that occurs between the chestburster and adult phases. As already stated, the Xenomorph appears to molt 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 Nostromo, Ripley comes across a food locker that had been raided, apparently by the Xenomorph to get food.

When the first Drone encountered on record was born in the Nostromo, it had a smooth, long head, and an upright-standing body. It had no visible eyes, a tail, and strange spines protruding from its back. However, many years later on LV-426, the same species was 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. This is supported by the fact that the alien aboard the Nostromo built a nest, which only Drones can do since it is part of their job in the colony.

Another widely accepted idea, however, is age - the Xenomorphs found on LV-426 were more than a few days old, while all of the others encountered were less than a day old. On the other hand, some hives seem to have a distinct coloration for each caste, in which the Drones are black and the Warriors are a very dark blue; the color difference indicating that age is not a factor.

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 variation between different populations, which evolve very fast.

Predalien
Like normal Xenomorphs, a Predalien gains traits from its host. 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. 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. Though an accepted breed among the Xenomorphs, the Yautja believe this kind to be abominations that must be vanquished by any means necessary.

Runner
Runners, a.k.a. "Dog Aliens", are produced from a facehugger jumping onto a dog or other quadrupedal species, such as an ox. They act as scouts for the hive, likely due to their quadrupedal and fast nature.

Runners are tinged a noticeable brown and run on four legs like the host they hailed from. However, they will walk and stand on two legs sometimes. On their back, there are no dorsal tubes, likely for better head mobility. Unlike most Aliens, they are digitigrade (i.e. they walk on their toes). Their chestbursters are not as larval as the other aliens, but rather a smaller, pale version of themselves.

The Runner is all about speed and agility, perhaps one of the fastest castes of Xenomorph ever encountered. It stalks its prey much like a lion or tiger, waiting for an unfortunate victim to come about and killing them outright, rather than cocooning them in a nest like others would. They are known to use stealth, just like their siblings. Although not as strong as other castes, Runners are still strong enough to lift a full grown Human. Like other Xenomorphs, they're also known to attack the head of their prey with their pharyngeal jawed tongues, and are capable of spitting acid.

Runners are shown to be more animalistic in nature and are not the sharpest caste in the hive. They do have some intelligence, but not as much as their bipedal counterparts. The Runner on Fury 161 utilized hit and kill tactics, rather than capturing creatures and cocooning them in an established nest. They are smart enough to not harm any members of their own kind, especially hosts impregnated with larval Queens. This was seen when Ellen Ripley was in the medical bay of the Fury 161 penal colony. After the Dragon killed Chief Medical Officer Jonathon Clemens, it quickly turned to Ripley, who was horrified of the creature and despite its intimidating moves towards her, left her completely unscathed. It even tried to protect Ripley from Leonard Dillon who was trying to get her away from it while it was trapped in a corner.

Conversely, this same individual has also demonstrated a desire for revenge, even against those carrying unborn Xenomorphs. This was seen after it was drowned in hot lead and managed to survive. This led to the creature chasing after Ripley until she dowsed it in water from the sprinklers, causing it to explode via thermal shock.

Culture and society
The Xenomorphs have a caste system similar to that of bees and ants. A major difference is that their caste system is far more complex.

It's believed that 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.

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. Also, they seem to have the job of creating their nests with their own secretions.

They average in size at about 6–7 feet tall (~ 1.8 to 2.1 meters) standing on hind legs, and about 14–15 feet long (~ 4.2 to 4.6 meters), tail included. They have the basic body build of the Warrior, except that their dome is completely smooth, whereas the Warrior's dome is typically 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 bodies that releases a sticky fluid that they can spit out of their mouth onto any surface. It can be used to harvest cocooned 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 (~ 2.4 meters) standing on hind legs, and about 14–16 feet long (~ 4.2 to 4.9 meters), tail included. In most cases, the Warrior looks the same as a Drone, except that the dome on top of the head is generally ridged.

They may evolve into Praetorians, which may serve as the Queen's personal guard and potential successors. Typically, there are four of them at any given time. However, when the Queen wishes to watch them battle for her attention 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
Praetorians ("Royal Guards"), 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.

The Praetorian is said to be created by special eggs and facehuggers known as a royal facehugger (these are generally larger and a darker color than normal facehuggers), which carries a genetic code known as the Royal Jelly Line, which is passed from the Queen herself to the facehugger and embryo. However, there are cases in which the 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 (~ 4.6 meters) and gaining its head crest. It returns to the hive as a worthy guard to the Queen.

Besides their guard duties, Praetorians may also act as the "princesses", or immature Queens of the hive, who can be promoted to Queens. The Xenomorph known as "Number Six" started out as a Warrior and demonstrated the ability to mature into a Praetorian and later a Queen. This rapid alteration is another evolutionary trait of the Xenomorph to ensure its very survival as a species. 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.

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

Queen
The Xenomorph Queen is the largest and most intelligent Xenomorph and can grow incredibly large, up to 100 feet tall (~ 30.5 meters) 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 varies. In one occasion, the Queen was shown to be maybe only twice the size of a Drone (this also may be due to her being so relatively young). In others, 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, as the Queen in question was very old: although kept in a dormant state for long periods of time, she could have been alive for several millennia. Alternatively, she could have been an Empress. Queens usually stand around 15 to 20 feet tall on average (~ 4.6 to 6.1 meters).

As stated, Queens are created through royal facehuggers which produce Praetorians, or immature Queens. When a Queen dies or abandons a hive, a Praetorian may fully develop into a new Queen and take her mother's place. Whereas normal embryos adopt characteristics from their host, the royal facehugger's does not. The royal facehugger carries within it the Royal Jelly Line, which is a genetic code passed down from the Queen Xenomorph to her chosen offspring. The embryo is more or less a genetic copy of its mother and as such will not adopt the characteristics of the host, but will retain the physical characteristics of the Xenomorph Queen. Regardless of host, the Praetorian will appear nearly exactly the same as its mother when it fully develops into a new Queen. Because of this, it's often speculated that the Queen is the only "pure blooded" Xenomorph variety.

Empress
There is mention of a Xenomorph Empress as well. Dr. Eisenberg might have merely used the term "Empress" to refer to the Queen. However, the Empress is bigger than the average Queen, and might also represent a distinct caste.

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 feet tall (~ 6.1 to 7.6 meters); and the fact that 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.

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.

Above the rank of Empress there is the Queen Mother: the single supreme ruler of all Xenomorph hives on all conquered planets; which resides on Xenomorph Prime, accompanied by a personal guard of Queen-sized Praetorians known as Palatines.

Other castes
Other Xenomorph castes are rarely seen, and it's possible that not all hives have them, and/or that they prefer to stay hidden for the most part, and avoid venturing out of the hive or facing intruders.

One documented caste are the Workers, a.k.a. Weavers or "Albino Drones", due to their white coloration. These Xenomorphs are considerably smaller than regular Drones, standing between 4 and 6 feet (~ 1.2 to 1.8 meters), and are usually not confrontational. More-so than any other caste, they're primarily concerned with the maintenance and expansion of the hive's structure, and may also tend to the Queen. They're most notable for their extremely developed proboscis-like inner mouth, which is used to secrete the resin to build up the hive walls.

The Carrier caste and the Ravager caste are both large, specialized forms derived from Praetorians. The Carrier's function is to harbor, protect and transport facehuggers, which will cling to the protrusions on its dorsal spines, and may launch themselves at nearby creatures. The Ravager, on the other hand, is a purely offensive caste which seeks only to destroy opponents, regardless of whether they could make useful hosts or not. They're only really employed when the hive is at war.

A particularly dangerous, but seldom-seen possible caste is the Crusher. This massive, tank-like, heavily-armored quadruped is mostly known for its wide bulletproof cranial shield. Despite being as large as a Praetorian, it's capable of achieving great speed, lending support to the idea that it might represent a Praetorian matured from the "Runner" (Xenomorphs spawned from quadrupedal hosts, such as dogs). Another possibility is that the Crusher might represent a mutation, rather than a naturally-occurring caste.

Other less prominent castes include the Lurker: essentially a Drone which, as its name indicates, specializes in stealth tactics. The poorly-known Boiler and Spitter castes were encountered by the crew of the USS Sephora on LV-426; as was the afore-mentioned Crusher. Although most Drones can spit acid, the Spitter possesses specialized pouches on its head to store additional quantities. The Boiler, meanwhile, is probably one of the strangest castes: a deformed-looking alien whose suicidal attack consists of rupturing its own body and "exploding" in a rain of acid.

Although a few artificially-created individuals, such as the Rogue Xenomorph, have been referred as "king", the existence of naturally-occurring Xenomorph Kings is dubious, supported only by apocryphal sources.

History
It is heavily implied that Weyland-Yutani's original synthetic cyborg David was the chief designer of the first Xenomorph egg and therefore the creator of the species. However, there are no solid facts as to the origins of the Xenomorph species; instead, there are many theories which cannot be confirmed. They may be an artificially created species, or they may have evolved naturally on a planet very different from Earth.

It has been implied that the Xenomorphs were created as biological weapons by the Engineers (born from Humans). To be specific, the android David (an android built to be curious and Human-like) infected Charlie Holloway with a drop of the mutagen and Charlie then impregnated Elizabeth Shaw; resulting in a large facehugger-like lifeform (known as the Hectopod or Trilobite). Elizabeth, in pain from the creature growing within her, then removed the lifeform by entering the automated surgery table on-board Meredith Vicker's exclusive ship. The lifeform later encountered an Engineer and ensnared him much like a facehugger; thus, creating the Deacon which highly resembles the Xenomorphs.

David was also shown to be the chief developer of the first Xenomorph eggs that contain the "facehuggers". Utilizing the now emptied planetoid Elizabeth Shaw and David landed on, David was able to combine the limited number of creatures and organisms available with the "Black-Goo" (not to mention Elizabeth Shaw's female reproductive system) to develop the first "facehugger" eggs. David knew he needed Human hosts to eventually gestate the Xenomorphs to fabricate his fantasy of becoming a god and creating something even better than his own makers. The Xenomorphs have come to be known as "the Perfect Organism".

Thus, the chain of "uplifting" can be summarized as the following.

--> Engineer --> Human --> AI Synthetic --> Xenomorph.

However, this is heavily contradicted by the fact that the Xenomorphs are known to be a pre-existing species that was traditionally bred, reared and hunted by the Yautja intentionally for sport: a tradition that goes back thousands of years and has been practiced on Earth as well as other planets. Unless the Yautja or another species has found a way to time-travel and brought Xenomorphs to the distant past, this remains incompatible with the idea of Xenomorphs having been genetically engineered by David.

The Xenomorph could be the result of genetic manipulation by the Mala'kak (also known as the "Engineers" or "Space Jockeys") as a terraforming mechanism about ten million years ago, which went terribly wrong. This theory doesn't quite account for the incident in which a Mala'kak presented containers filled with an unknown Black-Goo substance (known as "Chemical A0-3959X.91–15"). This chemical mutated harmless organisms into somewhat hostile creatures and. As explained above, when a male Human, infected with this substance, inseminated his mate, it produced a different subspecies of "hecto-facehuggers" (the so-called Trilobite) that could grow incredibly large and gave origin to the "Deacon" or "Proto-Alien".

Another theory is that the Xenomorph were the alpha predators of their own ecosystem on a nightmarish and harsh planet (assumed to be Xenomorph Prime, but there is no solid evidence). Without their native ecosystem to keep them in check, they developed into a separate dominant species. The Mala'kak, the Yautja, Humans and the Reapers began showing interest and started using them as means of sport, terraforming mechanisms, biological weapons, culinary delicacies (in the case of the Reapers and a giant unnamed reptilian race) and scientific experiment subjects.

Films

 * Alien (1979)
 * Aliens (1986)
 * Alien³ (1992)
 * Alien Resurrection (1997)
 * Alien vs. Predator (2004)
 * Aliens vs. Predator: Requiem (2007)
 * Alien: Covenant (2017)

Literature

 * Aliens: Earth Hive (1992)
 * Aliens: Nightmare Asylum (1993)
 * Aliens: The Female War (1993)
 * Aliens: Genocide (1993)
 * Aliens vs. Predator: Prey (1994)
 * Aliens: Alien Harvest (1995)
 * Aliens: Rogue (1995)
 * Aliens: Labyrinth (1996)
 * Aliens: Music of the Spears (1996)
 * Aliens vs. Predator: Hunter's Planet (1997)
 * Aliens: Berserker (1998)
 * Aliens vs. Predator: War (1999)
 * Aliens: Original Sin (2005)
 * Aliens: DNA War (2006)
 * Aliens: Cauldron (2006)
 * Aliens: Steel Egg (2007)
 * Aliens: Criminal Enterprise (2008)
 * Aliens: No Exit (2008)
 * Alien: Out of the Shadows (2014)
 * Alien: Sea of Sorrows (2014)
 * Alien: River of Pain (2014)
 * Alien: Invasion (2016)
 * Aliens: Bug Hunt (2017)
 * Alien: The Cold Forge (2018)
 * Alien: Echo (2019)
 * Alien: Isolation (2019)
 * Alien: Prototype (2019)
 * Aliens: Phalanx (2020)

Video games

 * Aliens versus Predator (1999)
 * Aliens versus Predator 2 (2001)
 * Aliens versus Predator: Extinction (2003)
 * Aliens: Extermination (2006)
 * Aliens vs. Predator: Requiem (2007)
 * Aliens vs. Predator (2010)
 * Aliens: Colonial Marines (2013)
 * Alien: Isolation (2014)

Tabletop games

 * Aliens Adventure Game (1991)

Comics

 * Dark Horse Comics