Standard Headcrab

"Have you ever seen such a magnificent species? These "crabs" can completely control their host's nervous system."

- Black Mesa scientist

Headcrabs, also referred to as "head humpers" by Barney Calhoun and "parasitics" by the Combine Overwatch, are the most numerous and arguably iconic aliens in the series.

Physical description
Headcrabs are fairly small creatures, consisting of a rounded, tan body with four reddish legs for movement and a pair of larger frontal claws for attacking prey or as additional support when standing still. Under the headcrab's body is a large rounded mouth surrounded by pointed flesh (which also includes the headcrab's stubby front legs) and a sharp, claw-like "beak". Headcrabs appear to have no external sensory organs, so it is assumed they move about by touch and vibration.

Headcrabs can produce a variety of vocalizations. When they are not hunting, headcrabs usually emit squeaks and fairly quiet, repetitive calls while gently swaying their bodies back and forth. When attacking, headcrabs emit a sharp, shrill shriek as they leap towards their victims.

In Half-Life, Headcrabs were found to have originated from a giant creature known as a Gonarch which shares similar physical qualities with headcrabs. Underdeveloped "baby" headcrabs spawn at a rapid rate from a sac that dangles beneath the creature.

Behavior
Although small, slow-moving, relatively weak on its own, and even seemingly harmless with its tip-toe like gait, the Headcrab has the ability to quickly leap long distances using its hind legs, while tilting upwards to face its mouth towards its target, inflicting minor injury with its claws, legs, and teeth. Its main goal, however, is to attach itself onto an appropriate host's head. Once attached, they take control of the host's nervous system and dreadful "mutations" occur, creating a "Headcrab Zombie"

A Headcrab will frequently lie and wait in dark corners or ventilation shafts for unsuspecting prey to walk by. In more open environments, the creatures are known to bury themselves in the ground to hide, then climb out to the surface and attack when a possible host draws near. Headcrabs appear to be fairly unintelligent creatures and will pursue their prey under the most dangerous conditions, moving through dangerous areas or towards hostile prey, even as fellow headcrabs die in front of them.

Headcrabs have been observed to be prey for Bullsquids, Vortigaunts, and Barnacles. Headcrabs have also been seen cooked and prepared in different dishes by Vortigaunt chefs in Black Mesa East, which suggest that they are used as a common food source after the extinction of most of earth's species. Citizens in Half-Life 2: Episode One comment that "They don't taste like crab."

It is also apparently possible to "tame" a headcrab to some extent, as demonstrated by Isaac Kleiner's "pet" headcrab, Lamarr.



Combine application
In Half-Life 2, the Earth-occupying Combine alien race is seen utilizing headcrabs as a form of biological weaponry against the Resistance. "Headcrab shells", as they are known, are shells containing several headcrabs. After impact, their payload of headcrabs is released into the open, free to infest or kill nearby victims. If used in large numbers, these shells are highly effective in neutralizing a large concentration of Resistance groups, as proven during a visit to the devastated town of Ravenholm.

Behind the scenes

 * The Headcrab was originally to be used as a weapon in Half-Life 2, mirroring the Snark from Half-Life, as part of the Brickbat weapon. This weapons was to include other props that could be picked up and thrown on enemies by the player (spawning the Molotov cocktail in the playable Half-Life 2 Beta will show the player holding a Headcrab, while it appears as a regular Molotov when thrown).
 * The Alien Assassin cut from Half-Life 2 was also to have its zombie version..
 * The Combine's mechanism for obtaining large numbers of headcrabs for use in Headcrab shells was revealed in Half-Life 2's 2003 code leak, in which a model of an amputated Gonarch sac could be found suspended in a Combine-built frame . Officially however, no reason has been given.
 * Curiously, although Half-Life 2's headcrabs are capable of tolerating areas contaminated by toxic waste and radioactivity, they are unable to swim and survive in liquid substances (including water); these traits are reversed in Half-Life. The headcrab's resilience towards toxic and radioactivity materials appears to be passed on to the hosts they parasitize, as zombies are frequently found in conditions that would otherwise be hazardous to their hosts.
 * From a post-it note in Dr. Kleiner's lab reminding him to find more watermelon for Lamarr, headcrabs are omnivorous.

Notes and references
Kopfkrabbe