“A facehugger is the second stage in the Alien’s life cycle. It has eight long, finger-like legs, which allow it to crawl rapidly, and a long tail adapted for making great leaps. These particular appendages give it an appearance somewhat comparable to chelicerate 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 legs around the victim’s head and wrapping its tail around the host’s neck. Upon making contact, the facehugger administers a cynose-based paralytic in order to render it unconscious and immobile. During a successful attachment, the facehugger will insert a proboscis down the host’s throat, while simultaneously implanting an embryo. The host is kept alive, and the creature breathes for the host. Attempts to remove facehuggers generally prove fatal, as the parasitoid will respond by tightening its tail around the host’s neck, and its acidic blood prevents it from being cut away. In addition, its grip on the host’s head is strong enough to tear the host’s face off if it is forcibly removed.”