Headcrab

A headcrab is a fictional alien species found in the Half-Life and Half-Life 2 computer game series created by Valve Software. Although small, slow-moving, and relatively weak on its own, the headcrab has the ability to quickly leap long distances, and its primary goal is to attach to the head of a human host, whereupon it takes control over its victim's body, rendering him a mindless zombie-like being. They are similar to the Facehugger aliens from the Alien movies and games.

Additionally, the headcrab's alien physiology causes various mutations to rapidly manifest in its host, such as elongated claws, increased strength, and a sharp-toothed, sideways mouth that bisects the victim's chest cavity from neck to groin (this last feature has caused some fans to dub the headcrab zombies as "Mawmen"). The results of these mutations may depend on the type of host and/or the length of time the headcrab is attached.

For instance, in Opposing Force, headcrabs have infected some of the Marines. In some of these cases, the result is a "normal" zombie, but in others, it results in a so-called "Gonome", a larger, stronger, more intelligent creature with a larger mouth in its chest cavity that produces some sort of volatile saliva the creature can throw. It is unknown if this variant is a result of a physically more powerful host, a longer gestation, or if it is an ability only certain headcrabs possess.

In most cases, a headcrab remains attached to its host until destroyed, although in Half-Life 2, they have the ability to detach from the host if it is damaged significantly. Removal of the headcrab reveals that the host's face has been largely eaten away by the creature, leaving behind merely a skull, eyes, and minor residual tissue.

Also new to Half-Life 2 are two previously unseen varieties of headcrab. One type has longer, thinner, almost spider-like legs, allowing it move much faster. This type seems to produce leaner, skinless zombies that are astonishingly fast, agile, and have the ability to leap and climb. The second type, the "poison headcrab" is identified by the white bands that encircle the area where the leg meets the body. To distinguish it further, its skin is a brown/black colour, with thick hairs on the joints of its long legs. It makes a very distinct and unpleasant sound similar to that of a rattlesnake's tail when it detects a viable host. Its bite injects an extremely powerful neurotoxin in its victim. It seems to produce a bloated, hunched-over "carrier" zombie that is usually encumbered with several other poison headcrabs hitching a ride on it. The zombie is able to hurl these headcrabs at enemies over a great distance.

Headcrabs appear to be quite low on the foodchain in their native Xen habitat, as both bullsquids and vortigaunts have been observed feeding on them.