Its bony finger-like legs allow it to crawl rapidly and its long spine-like 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.
Until a facehugger is disturbed by a living faunal organism that is larger than it, it lives in an egg, approximately one meter in height. When the egg is disturbed, the egg springs open, and the facehugger jumps out, using its tail to propel itself through the air and onto its victim's face. It constricts the hosts with its tail, cutting off their oxygen supply and inserting a proboscis of its own to feed the host oxygen, and also to implant a Xenomorph embryo using afore mentioned tube. Victims who are cocooned are probably not rendered unconscious, which may explain how they know they are impregnated (other than seeing a dead facehugger on the floor) and often ask to be killed.
The facehugger remains attached to the host's face until a Xenomorph embryo has been implanted. Larvae are laid in the bronchial tubes. The egg hatches, and the pre-larvae burrows into the victim's aorta, where it feeds on the nutrients, and absorbs the host DNA to supplement its own. This will become a Chestburster. At this point, the Facehugger has finished its part of the Xenomorph lifecycle, and falls from the victim's face, dying.
BiologyEditLike adult Xenomorphs, a facehugger has extremely corrosive body fluid. It will kill the host if it is forcibly removed before implantation is complete, either tearing off the host's face with its extremely strong digits or strangling the host with its tail. Provided it has not yet clamped it's legs around the host's head, a facehugger can be removed without harming the host, but its extreme strength and determination necessitates multiple humans or similar species to remove it. Once removed it will attempt to escape and hide in order to ambush another host. If there is a barrier between the facehugger and the host's face, such as a space suit visor, th facehugger will secrete some of its corrosive body fluid to dissolve through it. One of the many reasons that makes the parasite dangerous and feared other than the agonizing death it will eventually cause the host to have, is how fast they can subdue any host no matter how healthy or strong. Once a facehugger has fully attached there is no hope of removing it without surgery (even then, the host will likely die), though Ripley 8 was facehugged and managed to pull the facehugger off, though the mixture of her DNA with a Xenomorph Queen made her far stronger than any normal human.
Subduing a victimEdit
While a facehugger's tail could be used to strangle a host unconscious, it is unlikely it does it. Hosts are rendered unconscious within seconds, roughly around 7 seconds and the only choke that can knock someone out in seconds is a bloodchoke which stops the blood from entering the brain thus knocking a person out and requires a very precise chokehold. The other chokehold cuts off oxygen to the brain but usually takes longer to knock someone out. If the facehugger used its tail to render its host unconscious, then Ripley would have being successfully facehugged and unconscious well before Corporal Hicks and the others arrived in time to rescue her, as the tail was around her throat for more than 10 seconds and she was still conscious.
Its more likely or possible that the facehugger sedates its host through some sort of toxin or anesthetic. The reason why this is more likely is because of how fast the host is rendered unconscious and why the host would die if the parasite is prematurely removed, the sedation may render a hosts breathing function useless and the host is then completely dependent on it and so removing it would would cause the host to die from asphyxiation and not "murdered" by the parasite as its only concern is propagation of the species and nothing else.
A facehugger possesses a surprisingly strong grip, and those who are facehugged never manage to be able pull it off them before being sedated. A facehugger's legs may 'lock" once around a host's head, which would mean simply pulling it off will be nearly impossible and while a victim could try lifting it off like a helmet, its powerful tail prevents this. Its unknown if one could eventually remove a facehugger because victums are usually incapacitated within 7 seconds. In Alien it is implied and in Aliens it is stated surgery is required to remove a facehugger. In Alien, when Ash and Dallas attempt to remove the facehugger from Kane, there is mention that it is peeling his [Kane's] scalp with it. In Aliens, soon after they first get to Medical and see the facehuggers in tubes, Bishop reads out the chart for one of them where it mentions that the facehugger was surgically removed before embryo implanation but the host died during the procedure. Ripley 8 managed to do so but the circumstances were different as she was stronger than a human and was submerged in water. The facehugger would or could not sedate her while underwater presumably because either the water itself stopped its attempts or because once implantation was complete, Ripley would drown and the chestburster would die and chestbursters can only survive a host's death if it is ready to "burst" as before then they are still connected to the host. A facehugger's tail is also very powerful and strong, being described as very muscular, facehuggers use their tails to spring out from the egg in which they are born and also to help them attach to a host by wrapping their tail around a limb and then pulling themselves closer to the host.
In some cases facehuggers may stay alive for a short time after implantation, as seen in Alien when the facehugger has enough strength to reach the overhead light before falling on Ripley's shoulder. On Fiorina 161, a facehugger managed to impregnate both Ripley and a dog before dying.
Impregnating larger hostsEdit
For the size of a facehugger, humans are the best victims. However, a Space Jockey, which is significantly bigger than humans, is shown to have a ripped chest, implying that it has been facehugged. Prometheus reveals that the Space Jockey or Engineers are in fact humanoid and are smaller than what their suit makes them look.
Impregnating Smaller HostsEdit
In either a cut scene or script of Aliens, Ellen Ripley assumes a facehugger cannot impregnate anything as small or smaller than a cat. Dogs and similar sized animals are suitable and in theatrical release of Alien 3, a dog is attacked by a facehugger (in the special edition, it was a species of oxen) and births the first known Runner.
Behind the ScenesEdit
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. At first Giger assumed that the facehugger would wrap around the outside of the astronaut's helmet, but Scott decided that it would have far more impact if the facehugger were revealed once the helmet was removed. Scott and Giger realised that the facehugger could burn through the helmet's faceplate with its acid blood; subsequent redesigns of the space helmet included a far larger faceplate to allow for this. Dan O'Bannon initially conceived the facehugger as somewhat resembling an octopus, possessing tentacles. However, when he received H. R. 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 the legs 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.
In the special edition of Alien 3, a "super facehugger" is introduced. This would carry the embryo of the queen alien, but it was ultimately dropped from the theactrical version of the film. The super-facehugger is found by Murphy in the Assembly Cut of Alien 3, but not identified as such.