Facehugger

A facehugger is the second stage in the life-cycle of a xenomorph. Its bony finger-like legs allow it to crawl rapidly and its long tail can launch it in great leaps. These particular appendages give them an appearance somewhat comparable to Chelicerata arthropods such as  arachnids and  horseshoe crabs.

The facehugger is a parasitoid; its only purpose is to make contact with the host's mouth for the implantation process, by gripping its long, bony finger-like legs around the victim's head and wrapping its tail around the host's neck, then slowly tightening the tail in order to make the host gasp for oxygen and be rendered unconscious. By this point, the facehugger will have inserted a tube-like proboscis into the mouth and down the throat of the host, supplying the host with oxygen while implanting an embryo. Attempts to remove facehuggers generally prove fatal—the parasite will squeeze the host's neck with its tail, while the facehugger's acidic blood deters cutting it off. Not only this, but its grip is so firm, peeling it off would result in tearing off the host's face. In Aliens, a number of facehuggers are observed in stasis and accompanying medical notes indicate that at least one of the human hosts died during removal. Over time, a facehugger's outer epidermis becomes solidified and hardened by a chitinous layer of silicon. Once the alien embryo is safely implanted, the facehugger detaches and crawls away to die. Later, a larval Xenomorph, known as a Chestburster, will erupt from the host's chest.

Royal Facehugger?
There is much speculation as to how Queen Xenomorphs are born, many believe a royal facehugger plants a Queen embryo in its victim. Often times it is stated that a Royal Facehugger can implant several victims before it dies, and this is seen as the case for what occurred during the events of Aliens³.

Another possibility is the Queen Facehugger, which appeared as an action figure released by Kenner after Aliens³. However, these are stated as only being created as a comparison between the Warriors and Queen Xenomorphs, and are therefore very likely uncanon.

Behind the Scenes

 * In truth, the Facehugger's design is based off of human female reproductive organs (the mouth) and stereotypical human female fingernails (the claws at the end of each foot). This is why they make the joke about it in Aliens.
 * Giger's original design for the facehugger was a much larger creature with eyes and a spring-loaded tail. Later, in response to comments from the filmmakers, Giger reduced the creature's size substantially.  Dan O'Bannon initially conceived the facehugger as somewhat resembling an octopus, possessing tentacles.  However, when he received HR Giger's designs, which substituted tentacles with fingerlike digits, he thought Giger's design concept superior.  Since no one was available at the time, O'Bannon decided to design the facehugger prop himself.  The technical elements of the musculature and bone were added by Ron Cobb.  Giger's initial design for the smaller facehugger had the fingers facing forward, but O'Bannon's redesign shifted them to the side.  When the foam rubber sculpture of the facehugger was produced, O'Bannon asked that it should remain unpainted, believing the rubber, which resembled human skin, was more plausible.