Xenomorph

The Xenomorph is a parasitic life form with a multi-staged life cycle.

History
There is not one solid fact as to the origins of the Xenomorph species. Instead are many assumptions, which can neither ben confirmed nor denied.

The most commonly accepted theory is that they evolved naturally on a planet much different from our own.

Other theories state they were created by another race, perhaps the Yautja (Predator) as an "ultimate prey". Another theory in the form of scientific creation says that they were created by a race only to known to us as simply Space Jockeys as a biological weapon for the purposes we also don't know.

Vulnerabilities
Xenomorphs, in all stages of their life cycles, have been said to have vulnerabilities to heat, although this was never proven.

Xenomorphs are capable of adapting themselves to cold environments with little discomfort, though 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 an xenomorph's exoskeleton.

Xenomorphs are shown to be vulnerable to close range small-arms fire ranging from 9mm to 5.56mm 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.

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, language, 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 LV426 colony and operating an elevator. In the director's commentary for Aliens, James Cameron noted that the xenomorphs in Aliens had been alive for far longer than the alien in the original, and so had more time to learn how to manipulate machinery, if only at the most basic level. Xenomorphs have demonstrated little actual emotion, though they are not totally devoid of fear, especially for their eggs.

In the Aliens literature, it has been suggested that the creatures have a hive-mind, or a collective consciousness directed by the queen. In the Aliens comic book series, as well some novelizations (produced before the Alien³ film), it is suggested that the Queen alien 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 very similar to the Cthulhu cult in H. P. Lovecraft's The Call of Cthulhu.

Through cloning in the events surrounding Alien: Resurrection (noted in the novelization), it appears that the alien's 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 showed that the species excells at observational learning.

In the rare event that a xenomorph should find itself under attack or otherwise vulnerable, it once again displays incredible creativity and cleverness. They are quite capable of moving quickly 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, such as breaking out of human constructed confinement, or use it as a weapon.

Physical abilities
The adult xenomorph is basically 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 simply 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.

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 prey. They can survive in extreme temperatures, are well-adapted to swimming, can respirate 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 acidic, though not the the same extent as the blood, and 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 alien. 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, and as depicted in the Aliens vs. Predator games, the aliens can detect their prey using pheromones.

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.

Anatomy
The Xenomorph drone is equipped to be the perfect killing machine. It dons an exoskeleton much like that of a lobster, which is impenetrable to everything but the most powerful human fire arms (12-guage shotguns, Pulse Rifles, and stanchion launchers are quite effective). Attached at the ends both of it's arms are a set of claws capable of slashing through some types of metal (as seen in ALIEN books such as Book 2: Nightmare Asylum).

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 suggested, both on screen and in the games based on the Alien series, that the alien's form is affected by its host, so that different species of host will create different varieties of xenomorph. Variations within a host can also explain possible variations apparent in the xenomorph spawned. 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. It is also possible that there are different breeds of Alien that are not directly related, causing a difference in appearance. Furthermore, issues of the comic book and various versions of the Alien vs. Predator (video game) depict a "caste" system similar to ants and bees among the xenomorph hives (queen, warrior, drone, etc.), each with a different appearance.

Upper body
In the original Alien film, and in the sequel Aliens, they are depicted as tall, slender creatures with a roughly human biomechanical design. In the 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 aliens in the other films. In the comics and various video games (Aliens versus Predator: Extinction, Alien vs Predator (SNES), Aliens versus 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, but this is of course non-canon. In the movie there are only two castes of aliens, queens, and drones/warriors. The drone/warriors may have different tasks: Some defend the hive, others hunt potential hosts, while some live only to protect the queen.

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 the fusion of two digits. No explanation is given in the films for the fluctuating number of digits.

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 itself does not. This attribute is used later on in various games.

The Head
The most notable feature of the Xenomorph species is the elongated, almost banana-shaped head. Though it lacks 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 theory 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).

Other proposed theories 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 so the are not visible from the outside-in. 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 the smooth top working like a one-way visor. Another similar theory is that chestbursters might sense each other, likely through a telepathic hive mind, as was mentioned for how the Queen can keep track of her broodlings. They may in fact have skin-covered eyes and the ability to sense pheromones.

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. Some imagine these creatures to have come from a mainly amphious planet and perhaps they're a mechanism to swim under water faster to catch prey.

A less accepted theory is that because the creature has an exoskeleton it is unable to release body heat built up while chasing prey. To compensate for this, the spines along the back act as a ventilation system to release excess body heat from the creature to keep it from inadvertantly killing itself.

Another theory is that the spines are spiracles that take whatever is in the air around it and filters it in while expelling wastes. It is widely accepted that they do not have lungs so this theory is possible.

Secondary Jaws
The Seconday Jaws, also known as an Inner Mouth or Maw is a disturbing feature of the species. It's assumed that it's powered by extremely powerful muscles inside the monster's head, much like a human tongue is the single most powerful muscle in it's own body. This would hold true as it's been seen on numerous occassions, the alien being lashing this maw out, destroying anything it hits, including being 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 Tail
It is accepted that the tail is an offensive weapon and little more. Though the creature can and will run on all four appendages, it often stands on its two hind legs so it can reach for its prey with its arms. While standing it has also been seen to impale its victim with its tail, even to the point where it can raise a creature heavier than itself to eye level (if it had eyes), possibly in preparation for a face-bite using the secondary jaws. This would clearly be impossible if the tail were needed for balance, as the already shifting weight would be too awkward, causing the monster to fall.

Alien versus Predator (the movie) is not canon. The director did not study the Aliens enough to make sense out of the way they are.

Acidic Blood
The body fluid of all Xenomorph species appears to be a highly concentrated acid. It is mistakingly called blood, but when compared to the blood of every other lifeform known, this acid doesn't seem to carry any nutrients through the body. It is also highly unlikely it carries oxygen for two simple reasons: The first being that the Xenomorph has been known to survive in vacuums such as space, and the second being that when the fluid makes contact with the air, it's a chemical reaction with oxygen that triggers the corrosion and turns it into acid. 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 it's "blood" is, as one flicked it at a charging Predator. Some specially treated predator weapons can resist this acid, although how long exactly is unknown.

Genetic Adaptation
Adult Xenomorphs have been known to take on traits of their body host. How this is done exactly, is not quite understood. Through experiments, it was discovered that the embryo attaches itself to a major artery of it's host to get it's nutrients as it grows. It's presumed that the DNA from the blood it acquires infuses itself with the DNA of the embryo to make up for it's lack of genetic structure. Most of the time, there are only two forms it will take on. A thicker, bipedal form with a darker exoskeleton if it comes from something that stands on two legs, like a human. In the movie ALIEN 3, it came from a four legged animal, and thus took on a more slinder, 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 tusks when implanted in a Predator). Pred-Aliens (Yautja-Xenomorph hybrid offsprings) have a yellow-green partial skin covering their exoskeletons.

Life Cycle
The xenomorph has a unique multi-staged life cycle.

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 usually thick and gelatinous, usually with several membranes attached to the outer layers of the egg. The xenomorph hatchling inside the egg is protected by a thick fleshy layer at the top of the egg, where the "shell" peels open when the xenomorph either senses creatures nearby or wishes to seek food out.

In the first movie, it appears that they can also sense when another creature crosses into some form of mist that covers the nursery area. When this mist is broken the facehuggers appear to be alerted and go on the search for the host.

Facehugger
After the egg opens, the Facehugger as it is referred to, emerges from the egg and seeks out a host who is usually nearby and latches onto the host's face and implants the fluid essential for an alien to be born. Once the egg is implanted within the host the facehugger falls off and dies. This is the extent of the purpose of the "facehugger". Note: Whatever lifeform the facehugger impregnates the Xenomorph will take that form in its life.

ChestBuster
The second stage of the Alien life cycle is also known as the Chestburster. After gestating in the host's chest cavity, the xenomorph infants chew their way through the host's chest, causing severe pain, blood loss, and shock. All who have been "impregnated" by a xenomorph have died from this process. At this stage, the xenomorph is very vulnerable, and hungry. Therefore, the chestburster is needed to get to an area of small mammals (such as animals or infants, ideally) to feed and get to the next stage of life - typically shown as the warrior caste (although this is certainly not always the case) - as seen in Alien vs Predator : Extinction.

Adult
There have been a few variations of warrior aliens encountered. They are also known as Drones, Warriors and Adults.

Little is known about the transformation that occurs between the chestburster and adult phases, although the xenomorph appears to moult before reaching maturity. Maturity is reached in a matter of 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 alien to get food. Whether or not this was nourishment to grow was not specified. In the videogame, Aliens vs Predator: Extinction, the chestburster goes through a cocoon stage shortly after emerging from its host. A fully-developed xenomorph emerges from the cocoon afterwards.

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 looked slightly different. The head was no longer smooth, but had ridges. Why this is is unknown. The most widely accepted theory is age, as the Aliens found on LV-426 were more than a few days old, where all the others encountered were less than a day old.

another variation is the dog or runner alien caled so because of its dog like stance and higher speed .became this way because of its choice of host a cow giving it a stance on all fours

Another variation, occuring in the year 2004, was encountered by an expedition to Antarctica. These warriors were less humanoid, and sometimes walked on all fours. The tail was very long, with a large, curved, barb at the end. The head was smooth, but much shorter. It is important to note, however, that the particular alien "hive" found in Antarctica was grown by the Predators, who introduced a hormone with specifically unknown effects into the Queen's system.

Queen
A Queen is the largest xenomorph in a hive, and she lays many eggs which hatch to become the first stage of xenomorph. It is unclear whether the Queen of a hive engages in combat outside of protecting her hive. The Queen is usually nestled deep in the hive, protected by warrior guards. The queen has been known to display some logic-based intelligence, primarily in using simple human technology. She is a fusion of Termite queen and Bee queen in terms of threat level. When attached to Her egg sac, she is immobile, albeit ungodly frightening. When She tears out, however, She can move suprisingly fast. She attacks with Her four clawed arms, as well as Her long sting-tipped tail.

Alternate life-cycles
In Alien: Director's Cut, the xenomorph has a second method of reproduction, whereby it could transform humans into eggs, as shown when Ripley discovers Brett and Dallas, cocooned in a viscous liquid, with Brett almost completely enveloped by a distinctly egg-like mass. This method of reproduction allowed an alien a complete individual life-cycle, without the need for a queen. The alien was described in the bonus DVD as being "ambi-sextrous". However, since this scene was cut in the final released version of the film, allowing the queen to appear in Aliens, its canonicity is uncertain. The same way of reproduction was also presented in Alien Versus Predator game for Atari Jaguar. There, while playing as Alien, player could transform humans into egg-like cocoons, similar to that shown in the scene from the movie.

In another omitted scene from the script for Alien³, these eggs were actually cocoons, inside of which a human was painfully transformed into a full-grown alien, which then emerges from the cocoon like a perversion of a butterfly. This non-canon tertiary version of reproduction bypasses queens and facehuggers entirely. However, this scene was never filmed.

In Alien³, another addition was made. That of a 'super facehugger' that could impregnate two hosts with a Queen and a warrior embryo thus explaining why both Ripley and a dog were impregnated from one facehugger. This facehugger was large and black, very different to the normal tan variety. The super facehugger was found by some of the inmates, who thought it was a type of jellyfish.

Design
Though maintaining the basic design shown in the Aliens versus Predator video games, numerous artistic liberties were taken in it's film portrayal. One concept was to have the PredAlien reproduce by injecting victims with "DNA acid" through its tail. The injection would result in the victims swelling and exploding, creating a new PredAlien hybrid. Another abandoned concept was to have the PredAlien inherit it's hosts tendency to skin victims.