Xenomorph

The Xenomorph, or Alien is a perfect predator.

Biology
The Xenomorphs, in their adult form and otherwise, have many predatory advantages and distinguishing features such as extreme strength, very sharp claws, and a second mouth.

The head
The head is eyeless it is unclear how this creature senses prey. The head also elongated posibly to hold it's brain in the far back to make room for muscles for it's jaws and inner jaw (see below). Another notable feature on the Aliens head is the thick hard shell on top of it.

Secondary Jaws
The Seconday Jaws, also known as an Inner Mouth or Maw is a baffling feature of the Alien.

The Tail
While providing stability to the warrior, the tail is also discovered to be both a spearing weapon, and the ability to render an opponent harmless due to the strong neurotoxin in the tail.

Acidic Blood
It has been shown that the Aliens have acidic blood that can burn through most substances. This is mainly a defensive mechanism, although there are instances where this blood has been "spat" at prey.

Genetic adaptation
The Alien has been shown to take attributes of thier prey. A good example of this is the "predalien," an alien with the trademark mandibles of the 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.