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 distances. 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. However, it is also seen in the same film that it is possible to remove the facehugger if it is grabbed before it can make contact with the subject's face, although it takes considerable strength to pull it away. 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 dies. Later, a larval Xenomorph, known as a Chestburster, will erupt from the host's chest.

A facehugger can also tell if a person is an adequate host for the Alien embryo and will avoid people with serious health or genetic problems that could be assimilated into the new Alien. Facehuggers can also tell the difference between a living organism and a robot, as evident in an early draft of Alien resurrection. In said draft a Facehugger finds Call's unconscious body and is halfway through clamping on her face when it stops, places a feeler up her nose, and after sensing no actual breath scurries off to find a better host.

Facehugging Bigger Hosts
For the size of a facehugger, humans or humanoids (like Predators) are the best victims. However, a Space Jockey, which is significantly bigger than humans, is shown to have a ripped chest in Alien implying that it has been facehugged. Some fans doubt that a facehugger can constrain a creature as big as a Space Jockey.

As the Alien species has been referenced to ants from time to time, one theory is that facehuggers may attack in swarms like warrior ants so they can finally bring down and facehug large victims. Power in numbers. It is also possible that they could crawl down larger hosts throats and manually deposit the embryo there.

Behind the Scenes

 * The Facehugger's design is based off of human hands and genetalia.
 * 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.
 * Half-Life's (See Half-Life, PC Game) headcrabs resemble the facehuggers, as well as being in need of a host.