This article is about the species. For the most common subspecies, see Classic Headcrab.
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Headcrabs, often called "headhumpers" (or "head-humpers") by Barney Calhoun and "parasitics" by the Combine Overwatch, are the most numerous, and arguably the most iconic, aliens of the Half-Life series.
- Headcrabs are fairly small creatures, roughly the size of a pumpkin, which crawl on four legs, and have enough jumping power to leap up to 10 feet (3 meters) in any direction. Their underbelly is almost completely covered by a large, lipless mouth with a hidden "beak" used for feeding and latching onto the skulls of victims. (see below)
- As seen on Xen in Half-Life, Headcrabs appear to typically be born out of Gonarchs, therefore some Headcrabs may evolve into Gonarchs, and never use a host.
- During the Combine occupation of Earth, Headcrabs appear to have no external sensory organs, including eyes or ears, while the Classic Headcrab had 3 eyes on each side of its body 20 years prior.
- The Classic Headcrab can also be seen burrowing in and out of the ground, but not the other species.
- The Headcrab comes in several subspecies:
- The Classic Headcrab - the first Headcrab introduced in Half-Life, seen in its infant form near the end of the game. It turns its host into a Classic Zombie, and in some cases a Gonome.
- The Gonarch - a much larger, mature Headcrab species which spawns baby Headcrabs. Does not use hosts.
- The Fast Headcrab - introduced in Half-Life 2, and much faster than Classic Headcrabs. Turns its host into a Fast Zombie.
- The Poison Headcrab - a toxic Headcrab, causing more damage to the host. Also introduced in Half-Life 2, it turns its host into a Poison Zombie.
- Headcrabs can, apparently, be domesticated, as shown by Lamarr, Isaac Kleiner's pet Headcrab, who has been "de-beaked", and is said to be "completely harmless." Thus, removing the beak of Headcrabs renders them unable to perform their normal function of taking over humans, rendering them harmless and apparently less aggressive.
- It appears that the flesh of both Standard and Fast Headcrabs is edible, at least for Vortigaunts. The former can be seen being prepared by Vortigaunts in Black Mesa East while the latter can be seen being roasted by the All-Knowing Vortigaunt.
- Headcrabs are a parasitic species, attaching to the host's head and clamping onto their skull using their sharp beak. After breaking through the skull, the Headcrab proceeds to take over its host's motor functions through some unknown biological process, keeping their host alive but helpless through the entire process. As seen in Station 6 in Half-Life 2, it seems to take from several seconds to minutes for a Headcrab to turn humans into Zombies. As no humans live in Xen, the headcrab's homeworld, it would seam unlikely they they would evolve such abilities if there was no hosts. Therefore, it is very likely that they can host Xen creatures as well, such as Vortigaunts.
- The host appears to retain partial consciousness once the Headcrab has taken control, as Classic Zombies and Zombines retain a certain ability to talk.
- Each Headcrab subspecies affects its host in its own way, producing a Zombie extremely distinct from the other types, in both appearance and behavior.
 Combine application
In Half-Life 2, the Earth-occupying Combine is seen utilizing Headcrabs as a form of biological weaponry against the Resistance. "Headcrab Shells", as they are known, are large mortar shells containing a payload of Headcrabs, which are released on impact, free to infect or terrorize anyone nearby. If used in large numbers, these shells are highly effective in neutralizing a large Resistance base. The effects of large-scale Headcrab shelling are seen during Gordon's visit to the devastated town of Ravenholm, where most inhabitants have been turned into Zombies.
In Episode One it seems that the Combine have begun to lose control over their bio-weapon; Headcrabs begin infecting Overwatch Soldiers, showing that the large-scale effects of shelling have caused even Combine-controlled areas to be affected.
 Behind the scenes
- Originally, a device holding a Gonarch's sack, nicknamed the "Combine Big Momma Pod" a reference to the Gonarch's nickname, was to be used by the Combine for obtaining large numbers of Headcrabs for use in Headcrab Shells, and to be encountered in the Ravenholm mine. Since the device does not appear in the final games, it is unknown so far how the Combine breeds their Headcrabs.
- While the Headcrabs found in Half-Life and its expansions are able to swim and survive in water but not in areas contaminated by toxic waste and radioactivity, this is reversed in Half-Life 2 and its Episodes, where Headcrabs can be found surviving in toxic areas, and drowning when going into water. The Headcrab's new resilience towards toxic and radioactive materials even appears to be passed on to the hosts they infest, as Zombies are frequently found in conditions that would otherwise be hazardous to their hosts. This may be the results of their adaption to Earth's features, or modifications applied to them by the Combine.
 Half-Life and its expansions
 Half-Life 2 and its Episodes
 List of appearances
- Half-Life: Day One (First appearance)
- Half-Life: Uplink (Non-canonical appearance)
- Half-Life: Opposing Force
- Half-Life: Blue Shift
- Half-Life: Decay
- Half-Life 2
- Half-Life 2: Raising the Bar
- Half-Life 2: Episode One
- Half-Life 2: Episode Two