A Facehugger is the second stage of the Xenomorph's life cycle.
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 disturbed, the egg springs open slowly, and the facehugger jumps out, using its tail to propel itself in 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. Not all facehugger victims are rendered unconscious, Verheiden was still awake and trying to shout for help but couldn't due to the proboscis down his throat, host are probably not rendered unconscious if they are cocooned and it may explain why they know they are impregnated (other then seeing a dead facehugger on the floor) and asking to be killed.
It stays on the face until the Xenomorph embryo is inside. 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 is useless, and crawls off to die.
Like all Xenomorphs, they have a corrosive body fluid. It will murder the victim if it is successfully removed before implantation is complete, either tearing off their face with its extremely strong digits or strangling them with its tail. The facehugger will still be alive, searching for a new host, after its removal, as well. It can be removed without harming the host but its extreme strength and determination means it may take multiple humans or similar species to do this. Facehuggers take some of the host's DNA, about 10 or 15%, and incorporate them into the chestburster, so the xenomorph will be far stronger than the host as well as adapted as well as possible to the local environments. One of the many reasons that makes the parasite dangerous and feared other than the painful death it will eventually cause the host to have, is how fast they they can subdue any host no matter how healthy or strong. In AVP-R the first homeless person is rendered unconscious in 7 or 8 seconds and a Predator who is more muscular that those previously seen is easily subdued, it should be noted that the facehugger caught him off guard but it should also be said that Yautja are extremely strong (superhuman strength by Human standards). It shows that once a facehugger has fully attached there is no hope of removing it and the host-to-be's fate is sealed, though in Alien Resurrection, Ripley was facehugged and managed to pull it off, though she is far stronger than a human and she was submerged in water.
Subduing a victimEdit
While a facehugger's tail could be used to strangle a host unconscious it is unlikely it does it, host 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 hicks and others arrived in time as the tail was around her throat for more than 10 seconds and she was still conscious. It has been shown in films and some comics (canon status questionable) that people who get facehugged are unconcious almost instantly, An example would be the two homeless people. one was rendered unconcious in around seven or eight seconds and the other was unconcious within two seconds once it attached to him, these examples are when the parasite has fully attached. Also a Predator would easily remove a facehugger from it's face due to their black neck guards and even if it were ineffective, their extreme strength means they would easily be able pull them off. It should be said that most victims that get attacked usually try and get the parasite off their face rather than trying to uncoil the tail around their neck though some have been seen trying such as Roussea indicating that the facehugger is either not choking them or at least severely as anyone getting choked would panic and immediately try to pull at the tail but as just said, most hosts try and get the parasite off their faces. it is possible that the facehugger could and perhaps does use its tail to render someone unconscious.
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 may appear to be nothing more than a big spider to some but their strength is surprisingly strong, those who are facehugged never manage to be able pull it off them before being sedated, even Predators with their strength are apparently unable to remove the parasite from their faces in time. A facehugger's legs may 'lock" once around a host's head, which would mean simply pulling it off will be nearly imposssible and while a victim could try lifting it off like a helmet, its powerful tail prevents that. Its unknown if a human or Predator could eventually remove a facehugger because said species are usually incapacitated within 7 seconds. 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 uses their tails to spring out from an egg and also to help them attach to a host by wrapping thier 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 and tried to impregnate Ripley (although it only landed on her shoulder) before it died and in Alien 3, when a facehugger impregnated Ripley and then a dog (ox in the Assembly Cut).
Impregnating larger hostsEdit
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.