Native to the caverns and secluded areas of the Half-Life series' fictional planet / nebula / asteroid cluster of Xen, headcrabs are the "worker ants" of a wide-spread colony-based species (also named headcrabs, after their worker faction) that form a large chunk of your enemies in the video game.
Headcrab Caste Overview
The most common caste of headcrab, the headcrabs themselves, look vaguely like an uncooked chicken, only with four sharp insectoid limbs and a cavernous fanged mouth on the underside. They are approximately 2 feet in length.
On either side of the bulbous central segment there are two limbs, positioned in a thumb-and-forefinger arrangement. The front limbs are long and pointed spikes, with a small hook projecting from the underside of the tip. the rear legs are much shorter, and have two inwards-facing toe-hooks. Near the front of the creature, on the underside, there are two stubby mandible-spikes. Headcrabs mostly use their shorter back legs for locomotion, holding their front legs in the air, and resting their front end on these mandibles. When attacking, the clumsy but powerful front legs snap down and back, launching the creature forwards with an great deal of force and sudden speed. In this manner, the headcrab leaps at the victim's skull and will attempt to dig its hooks into any skin that would offer a firm grip. Often, the victim will die of these lacerations and they will eventually begin to merge with the headcrab and become a zombie (further described in the 'species overview' section and the 'zombie' node).
Headcrabs are most commonly found in complex systems of tunnels, which limit the movement of any prey and allow for easier attacks. They usually take advantage of pre-existing tunnels, but headcrabs also have the limited ability to burrow in softer terrains, like sand.
Headcrabs are also often found in large groups, but they exhibit no pack behaviour of any sort, as their groupings are mostly a result of mutual point of genesis and common preferences of habitat.
The natural predator of the headcrab is the bullsquid. The natural prey of the headcrab is unknown as they have only been observed to attack humans, which are not native to Xen.
Headcrab Species Overview
The rest of the species varies in shape, size and colouration, but they are all more or less arachnid and are four-limbed.
The "queens" of headcrab colonies are called gonarch: large shelled monsters that continously produce baby headcrabs from their pendulous egg sac.
These baby headcrabs will eventually grow up into one of the different castes of headcrab:
The first game contained only the gonarch, baby headcrab and generic headcrab castes, but the sequel (Half-Life 2) reportedly features four new types:
a faster, sleeker headcrab,
a poisonous beadcrab,
a larger headcrab
a flying (!) headcrab.
All headcrab castes (except the gonarch and baby headcrab) have the instinctive ability to latch onto the nervous systems of sentient species (hence the "head" part of the name), gaining control of their bodies and eventually mutating them into a half-alien zombie.
Normal Headcrabs will produce a shambling zombie.
After two days of feeding, these zombies will, in turn, grow into a much bulkier zombie, called a gonome.
Very little is known about the types of zombie that the newer headcrabs will produce, as Half-Life 2 has not yet been released. Updates will be added when it is.