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.

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).

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 believed that it "sees" by sensing phermones. 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).

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.

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). 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.

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.

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).

Life Cycle
The xenomorph has a multi-staged life.

Egg
The xenomorph is born in egg form. 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. Alien

Stage 1
After hatching, the hatchling, or Facehugger as it is referred to, seeks out a victim, and latches onto his/her face and implants yet another egg. Generally, once the egg is implanted within the host, the afflicted person awakens and the said facehugger falls off, scuttles off, and dies. The host generally looks and acts fine, but soon discovers chest pain and eventually, the xenomorph works it's way to it's next phase of life.

Stage 2
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 final stage of life, the warrior.

Stage 3
There have been a few variations of warrior aliens encountered. They are also known as Drones, Warriors, Soldiers, and Adults.

When the first warrior 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.

In another anomaly, on the prison colony Fiorina 161, a warrior alien was born with a smooth head, a short tail, no dorsal spines, and walked on all fours. This is generally believed to have happened due to it's choice of host: a four-legged animal.

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.