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 endoparasitoid species with multiple life cycles, possibly originating from the planet Proteus. 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 Facehugger implants. The human phenotype is generally around 7–8 feet 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 shown to grow even larger, some surpassing 100 feet tall if given time) and possess superior speed, strength and intelligence than their offspring.

Biology
The physical form of a Xenomorph can vary depending on its life stage, hive, caste, age, and host, as well as varieties which have been genetically engineered by host species or genetically grown. The typical Xenomorph start its life cycle as an egg-like creature which, when detecting the presence of a potential host, opens up to release the second stage: a small quasi-arachnoid life form (Facehugger) which launches itself onto the hosts face and impregnates it with an endoparasitoid larva. This parasite then consumes the host from within and exits the body as a vaguely worm-like organism, before growing very quickly and developing into an adult Xenomorph.

Although numerous varieties exist, adult Xenomorphs all tend to share some common traits: a vertebrate-like body, a protective exoskeleton, a long flexible tail, an elongated domed head, with nearly invisible eyes near the mouth, and a toothed maw hiding a proboscis-like inner mouth. 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 Fluoroantimonic acid type of fluid. It is dull yellow in color and often referred to as "blood", although whether it's used to transport nutrients through the body and/or just serves simply as a defensive mechanism is not clear.

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

Intelligence
Xenomorphs appear to possess a roughly similar intelligence to that of a typical primate, but have shown to learn relatively quickly, utilizing the situation around 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 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 therefore had more time to learn how to manipulate basic-level machinery. However, the genetic Xenomorphs from Alien: Resurrection learned very quickly that they were being contained, so they intentionally murdered the weakest so the acid of the Xenomorph's blood would eat through the floors, thus supporting the idea of fast learning.

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

A Xenomorph Warrior is equipped to be the perfect killing machine. They possesses an exoskeleton much like that of a lobster, impenetrable to everything but the most powerful human firearms (12-gauge shotguns, Pulse Rifles, and stanchion launchers are quite effective). Ash claims the creature's skin is made from 'protein polysaccharides', so this would imply a substance similar to chitin. He also says the creature replaces its surface cells with polarized 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, as shown in Alien 3, when these creatures are destroyed by first immersion in molten lead and then sudden cooling with fire sprinklers, causing its silicon shell to shatter.
Attached at the ends of both of their arms are a set of long fingers and shiny, black, and extremely sharp claws capable of slashing through almost anything within its grasp (as seen in ALIEN books, such as Book 2: Nightmare Asylum). They have two pairs of finger-like digits and appears to have two opposable thumbs (on opposite sides of the hand) on each hand. Its tail is long, and can be used as a whip. Later movies show them to have ridges on top, and a serrated toxic tip at the end to lacerate enemies.
They stand at around 7 feet tall, averaging in anywhere between 140 kg and 180 kg; although size may vary wildly depending on the host chosen, as seen for instance with the "Jock-Xenomorph". Their mouths is filled with sharp teeth, but their main killing device is the inner jaw that shoots out to penetrate the victim's brain.

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

[b]Physical Abilities[b]
The adult Xenomorph is a living weapon,(tail is used as 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 its tail to impale whoever walks by. The prey is generally blind to the fact that a Xenomorph is present, due to its propensity to camouflage itself within its nest walls or the surrounding artificial environment given their bio-mechanical-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 Earth life form. An example of this is the Queen in Aliens survived a multi ton exo-suit falling on her from over 30 feet. Xenomorphs are capable of surviving with limbs shot off as seen in AVP3 the game and Aliens. Even stabbings and shots through the head (though total decapitation kills them) as seen in Alien vs Predator and its sequel are not guaranteed to kill Xenomorphs.

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

The creatures possess no visible eyes; Giger mandated this in his original design, because he felt that it made the creatures much more frightening if one could not tell they were looking at them. In the Alien vs. Predator series, the game implies that the Xenomorph sees using its attack tongue, though this may be for gameplay purposes rather than actual speculation. 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 Alien³, a fish-eye lens (which creates a perspective similar to that of a peephole) was used to depict the Xenomorph's sight. Whether this can be considered sight or just a filmed representation of the creatures' sensory perception is unknown.
However, in the novelization of the movie Alien, the creature is held mesmerized by a spinning green light for several minutes. In the 2010 Alien vs. Predator game, it is mentioned on one of the Marine audio tapes that Xenomorphs have vision.
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. Xenomorphs may be able to detect their 'prey' through electro-reception. This is the method sharks use to detect even the most hidden prey. Creatures with the electro-reception sense can detect the electromagnetic field all animals produce allowing them to detect the creature's heart beat. This would explain how Xenomorphs always know where the humans are. And, as depicted in the Aliens vs. Predator games, the Xenomorphs can also detect their prey using pheromones.
Another theory is that the eyes of a Xenomorph are behind their black carapace and they see through it, similar to a one sided mirror (except in this case no one can see in, but the Xenomorph can see out). Some species of terrestrial fish such as the Barreleyes see through a transparent layer of skin; it is possible Xenomorph vision is similar.
Xenomorphs can also produce a thick, strong resin, which they use to build their hives and cocoon victims. Much like termites, they mix their viscous saliva with solids. It shows amazing heat and moisture-retaining qualities.
Vulnerabilities.

Xenomorphs, in all stages of their life cycles, have been said to have vulnerabilities to heat, although this was only shown in a deleted scene from the Alien Special Edition DVD, in which Ripley searches for her missing shipmates in the bowels of what appears to be the engine/boiler room of the Nostromo. It is also mentioned in the game AVP 3 by a flamethrower-carrying marine: "Buggers don't like fire. This'll flush 'em out". In the Alien: Isolation (2014) game, the only way to effectively drive off the Alien is with the use of molotovs and the flamethrower, however this causes it no actual damage (the Alien is invincible for gameplay purposes), and simply causes it to flee in a fearful panic.
The only Xenomorph not vulnerable to fire is the Predalien. In the game Alien vs. Predator: Extinction, it secretes a layer over their body. That layer cannot be ignited, though they are visibly pained by temperature extremes such as liquid nitrogen.
A rapid succession of extremes in both high and low temperatures apparently causes a thermal shock effect on a Xenomorph's exoskeleton. Xenomorphs are capable of adapting themselves to cold environments with little discomfort. The alien queen is extremely vulnerable to fire, for unknown reasons.

Xenomorphs are shown to be both invulnerable and vulnerable to small firearms, ranging from 9mm to 5.56mm. This is seen in one scene in Aliens where Vasquez and Lt. Gorman find themselves trapped in the ventilation between advancing aliens coming from both directions, Gorman fires with his pistol on one of them with no effect at all, instead it looks like they bounce off (presumably from the head carapace). In an earlier scene, Vasquez holds an alien at gun point and fires her pistol at the head, which apparently goes through and the alien rattles away, obviously damaged. High-caliber firearms, with the explosive rounds of a M41A Pulse Rifle may cause the creatures to detonate. High-energy beam weapons, such as the plasma weapons used by Yautja, appear to be the best way to kill the Xenomorphs, 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.
They are not invulnerable to melee weapons, although such equipment is likely to be destroyed upon contact with their acid blood.