Standard Zombie

A headcrab's primary goal is to attach to the head of a suitable host using its mouth (typically covering the face, and most of or the whole of the head), and burrow its claws and hind legs into the host, whereupon it takes control over its victim's body, rendering him a mindless zombie-like being.

The headcrab attaches itself on top of the head, where it opens up portions of the skull with its mouth (there is no animation showing a headcrab zombie with the entire skullcap missing, so it is assumed that the headcrab does not actually take off the skull covering). By incorporating parts of its biological workings with the motor cortex of the host's brain, it is able to utilize all of the motor functions of the host. However, the host's thought processes are seemingly undisturbed, and therefore it is assumed that the host is actually alive while being possessed by a headcrab. One could assume this due to the amount of screaming that is done by the infected host, which is muffled at all times by the presence of the headcrab.

Interestingly, no Xen species are seen infested with headcrabs. Indeed, the physical makeup of the headcrab, and its apparent need to clamp on the "head" of a humanoid makes it unlikely that it is capable of "zombifying" other creatures of Xen. As such, it is unclear how headcrabs subsist in their native environment, and even more unclear how this ability evolved. Lamarr, Dr. Kleiner's pet headcrab, is fed watermelons, although she is also seen to attack a crow later on in the scene, which suggests that headcrabs are omnivores.

The headcrab is vaguely similar to the facehugger in the Alien movies, although their purposes for their hosts are different. The headcrab's parasitic ability is also speculated to be inspired by the capability of the Emerald Cockroach Wasp to paralyze and control "zombie" cockroaches, in a manner that could be similar to how a headcrab controls its host.

Similar plot devices are used though fiction such as in Ian Livingstone's fighting fantasy book Island of the Lizard King, involving a creature called a "Gonchong" whose proboscis would burrow into the head of a host to control it, Clark Ashton Smith's short story The Vaults of Yoh-Vombis features slug-like creatures that eat away human skulls, then control their victims through neural stimulus; unlike the "Gonching", the headcrab does not actually bury into the skull of the host. In the Star Trek film The Wrath of Khan, Ceti eels are used. In all these examples an animal is used to control the thought and or actions of a human host.

Like standard headcrabs, both fast headcrabs and poison headcrabs are also capable of attaching to a host's head, although these headcrabs each induce different forms of mutations on their hosts and the resulting zombies employ different strategies of attack. An unusual characteristic of both zombie variations is that only three middle fingers of the five on each zombie's hands develop into claws, compared to all the fingers on a standard headcrab zombie. In addition, neither two variants have a describable "maw" nor the chest cavity that normal headcrab zombies possess, although fast zombies do have cracked rib cages and seemingly no internal organs, creating a hollow space.

Standard headcrab zombie


When a headcrab successfully attaches to a host, the host becomes a "headcrab zombie", or "necrotic", as it is referred to by the Combine Overwatch. 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 giant mouth has inspired the nickname "Mawmen". It should be noted that in Half-Life 2, their appearance is different; instead of this chest "maw", they appear to be protruding ribs: a ripped open chest cavity and no sternum. The official Prima game guide for Half-Life 2 still mentions the "Mawmen" nickname, however. The zombies do not appear to retain any of their former intelligence, blindly pursuing their prey, oblivious to danger. In Half-Life 2 they gain the ability to throw objects towards the player by hitting one that is in direct contact with the zombies; a small number of headcrab zombies would also survive despite losing both legs and its lower torso, crawling and moving using its arms. An overlooked effect that signals a zombie's presence in the run-down areas of City 17 is the buzz of flies, showing that the corrupted body of the victim is rotting.

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 wounded significantly. Removal of the headcrab reveals that the host's face and head has been eaten away by the creature; the degree of such damage on these areas depends on the type of headcrab in control. In Half-Life 2, horrified sounds emanating from headcrab zombies imply that, while severely wounded, the host creature is at least partially aware of its horrific situation. Playing parts of a victim's sound file backwards produces extremely disturbing yells of pain. The reversed cries have been interpreted as "God help me" and "get it off me".