Xenomorph

The Xenomorph XX121, better known just as Xenomorph or the "Alien", are an extraterrestrial, endoparasitoid species with multiple life cycles, originating from the planet Proteus. One of the deadliest of all known alien species, these creatures need a host organism in order to reproduce. The appearance of the Xenomorph varies depending on the facehuggers host. 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 (they can grow even larger, some even up to 100 feet tall, and stronger if given time) and possesses superior speed, strength and intelligence.

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

Although numerous varieties exist, the adult Xenomorphs all tend to share some common traits: a vertebrate-like body, a protective exoskeleton, a long flexible tail, an elongated 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 yellowish green 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 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.

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

Abilities
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.

Cocooning
Cocooning is a trait used by Xenomorphs to reproduce quickly. A human, or any other creature, if dragged away, would have a cocoon of saliva and resin placed over them (their face left free), usually in an area where the Queen has produced a large quantity of eggs, so that the Facehuggers are able to latch onto them easily and deposit a Xenomorph larva in their chest.

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

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

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

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

Hosts usually fall into a short coma and wake up ravenously hungry. This can be explained as the embryo needing an immediate source of nutrients needed for its progression into the next stage of its life. Hours might pass between the host waking and their time of death.

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.

Adult
The Xenomorph appears to moult before reaching maturity. Maturity is reached in a few hours, and involves a dozen-fold increase in mass, which would presumably require some form of nourishment.

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

Drone
The basis of any Xenomorph hive; the Drones are very much like worker bees or ants. Their main tasks are to keep the hive in good condition. 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 standing on hind legs, and about 14–15 feet long, tail included. They have the basic body build of the Warrior, except that their dome is completely smooth, where as the Warrior's dome is more ridged. Drones are smaller and somewhat more delicate than the Warriors, not considered quite as dangerous by Yautja standards, but striking fear into the heart of any human. Like all of their brethren, they have acidic body fluids, but only the Drone can spit this secretion onto prey over a short distance. Drones have an organ in their body that releases a sticky fluid that they can spit out of their mouth onto any surface. It can be used to harvest hosts for the Queen's Facehuggers.

Warrior
Warrior Xenomorphs - commonly known as simply 'Warriors' - are the soldiers and protectors of the hive. They are all children of the hive's Queen, bigger and stronger than the Drones. The Drones are less deadly, and somewhat less intimidating. Warriors average in size at about 8 feet tall standing on hind legs, and about 14–16 feet long, tail included. In most cases, the Warrior looks the same as a Drone, except that the dome on top of the head is completely ridged. They evolve into Praetorians, which serve as the Queen's consorts. Typically, there are four of them at any given time. However, when the Queen wishes to watch them battle for her attentions or increase protection in preparation for an attack on the hive, she will release pheromones which cause four additional Warriors to evolve into Praetorians.

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

Queen
The Queen is the largest and most intelligent Xenomorph and can grow incredibly large, up to 100 feet tall if given time. She lays many eggs that hatch to become the first stage of Xenomorph. It is unclear whether the Queen engages in combat outside of protecting her hive. The Queen is usually nestled deep within the hive, protected by Praetorian guards. She has been known to display some logic-based intelligence, primarily in using simple human technology such as lifts. She is the most dangerous of the hive, displaying extreme aggression and using her intelligence to devastating effect. When attached to her egg sac (ovipositor), she is immobile and vulnerable to attack. When she removes herself from it, however, she can move surprisingly fast. She attacks with her four clawed arms, as well as her long, bladed tail. She has also been shown to use her main and inner jaws in combat.