A headcrab is a fictional alien parasitoid found in the Half-Life video game series created by Valve Software. They are the most numerous and arguably most iconic aliens in the series. They are also sometimes referred to as head humpers or head hoppers by Barney Calhoun and some of the citizens in the game.
Headcrabs are parasitic lifeforms roughly 2 feet (0.61 m) long. Headcrabs have rounded bodies with four legs for movement; two long clawed legs at the front, and two stubby legs at the back. Their pair of large frontal claws are for attacking prey, and as additional support when standing still. Under the headcrab's body is a large rounded mouth surrounded by pointed rigid flesh with sharp claw-like beaks.
The design of the headcrab changes between Half-Life and Half-Life 2. Half-Life headcrabs have exposed teeth and intestines at their mouth, while the Half-Life 2 version does not. As such, it is initially apparent that the headcrab only walks with its large front claws and hind legs, while the stubby legs form part of the headcrab's mouth. The headcrabs found in Half-Life are also noticeably smaller than those in Half-Life 2, which have bodies approximately the size of a watermelon. Half-Life and Half-Life 2 also oppositely depict the headcrab's tolerance to water and toxins: headcrabs in Half-Life can swim in water but die in toxic environments, while headcrabs in Half-Life 2 drown within seconds in deep water but easily survive in contaminated pools.
Physically, headcrabs are fairly frail, a few bullets or a single strike from the player's melee weapon being sufficient to dispatch them. They are also relatively slow-moving and their attacks inflict very little damage. However, they can leap surprising distances to catch their prey. Headcrabs seek out larger human hosts, which are converted into zombie-like mutants that attack any living thing they can find. The converted humans are far more resilient than an ordinary human would be and inherit the headcrab's resilience toward toxic and radioactive materials. The games also establish that while headcrabs are parasites that prey on humans, they are also the prey of the creatures of their homeworld. Bullsquids, vortigaunts, and barnacles will all eat headcrabs (it should be noted that the barnacles will eat whatever they can grab); vortigaunts can be seen cooking them on several occasions.
Headcrabs are often found in dark areas, usually lying in wait in dark corners and ventilation shafts. In more open environments, headcrabs bury themselves in the ground to hide, then climb out to the surface and attack when a victim draws near. Headcrabs are often seen clustered in groups to make up for their individual weakness. They pursue their prey with single-minded determination, never fleeing despite the death of fellow headcrabs or any other dangers. Conversely, however, headcrabs attached to a human may abandon their host should it be severely wounded.
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 until they have sight of a host. When attacking, headcrabs emit a sharp, shrill shriek as they leap toward their victims.
While the original Half-Life has only one type of headcrab (not counting the Baby Headcrab), identified by its mottled tan-colored skin and reddish legs, Half-Life 2 introduces two more versions, the fast headcrab and the poison headcrab. These versions are more dangerous than their ordinary counterparts.
The fast headcrab is a faster, more spider-like version of the ordinary headcrab. Its skin is slightly lighter, and it has long, spider-like legs that allow it to move much faster and climb on walls (first demonstrated in Half-Life 2 in Ravenholm). It does not have a beak like the other varieties, instead using the sharp talons at the ends of its legs to latch to hosts. The zombies it creates are stripped of most of their flesh and muscle and, like the headcrab itself, are much faster than ordinary zombies. It also makes the same shriek that their standard cousins make.
The poison headcrab (also known as the black headcrab or venomous headcrab) is slightly larger than its counterparts with dark sage-green-colored skin (sometimes with a wet shine, attributable to a lighting problem with Valve's Source engine) and thick hairs on the joints of its inward-bending legs. To distinguish it further, it has white bands that encircle its knee joints, wider legs, and a more flattened body, giving it a generally more crab-like appearance. It also makes a chirp at range and a hissing-rattling similar to that of a rattlesnake's tail when it detects a viable host. Immediately before pouncing, they emit a loud, distinctive shriek. There is also a whipping noise as they pounce. Unlike the other headcrabs, the poison headcrab has some survival instinct, as it will retreat when injured. Although it is the slowest-moving version of the headcrab when calm, it can outpace an ordinary headcrab when retreating. Another difference is that the poison headcrab takes a longer time to incinerate than the other types.
Poison headcrabs get their name from the neurotoxin they carry, which reduces the player's health to one point instantly on contact. Gordon's HEV suit provides an antidote that will restore the missing health over a short period, minus the damage caused by the attack itself. While this makes the poison headcrab unable to fully kill Gordon, it can make survival much more problematic if other enemies are present. Half-Life 2: Raising The Bar notes that play-testers would prioritize poison headcrabs as targets, regardless of any other present dangers. They will group together on a single host once one is found: the attacking poison headcrab controls the host, while the others use the new host as transportation, having it throw them at new victims.
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 the head). The headcrab then burrows its claws and hind legs into the host and opens up portions of the skull with its mouth, incorporating parts of its biological workings with the motor cortex of the host's nervous system. The victim is thus taken over by the headcrab and mutated into a mindless zombie-like being known as a headcrab zombie, referred to as a "necrotic" by the Combine Overwatch.
The headcrab's alien physiology causes various mutations in its host, giving it massively oversized claws, increased strength, and what appears to be a sharp-toothed "mouth" that bisects the victim's chest cavity from neck to groin. In Half-Life 2, their appearance is slightly different; the "teeth" of the mouth are clearly revealed as protruding ribs: a ripped open chest cavity and no sternum. In Half-Life, zombies can be seen tearing flesh from corpses and feeding it into their "mouth". In Half-Life 2, headcrab zombies are capable of surviving even if they are severed at the torso, simply dragging themselves along the ground with their arms. Headcrab zombies signal their presence through various muffled groans and grunts, sometimes mixed with agonized screams. Several players have observed that although the screaming of the zombies merely sounds like nonsense, when reversed (But some players say its not reversed, you just have to listen closely), they appear to be screaming in terror, Breathing very heavily as if in intense pain and praying to God to "kill them".
Like standard headcrabs, both fast headcrabs and poison headcrabs are capable of attaching to a host's head, although these headcrabs 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 standard headcrab successfully attaches to a host, the host becomes a standard headcrab zombie. The torso of the host is open and the organs can be seen. In Half-Life 1 the player can see the texture of the host's skull on the headcrab. However, this is removed in other installments. They are slow-moving but powerful, using their claws to beat their victims to death. They moan almost constantly, and growl when they detect prey, but in "Half-Life 1" the zombies seem to not detect you in some instances. Because headcrab zombies are slow, they are often found in confined spaces and/or in large numbers to compensate. In addition, standard headcrab zombies will often feign death until the player approaches, catching them off-guard. In Half-Life 2, they can swat loose objects on the ground when they run into them, creating potentially lethal projectiles. Half-Life 2 also introduced still-moving zombies which are severed at the waist; these half-zombies crawl toward the player using their arms.
The gonome is apparently the next step of a standard headcrab zombie's mutation. It is larger, stronger, and more intelligent than "ordinary" zombies; it also produces a stomach acid that the creature can retrieve from its chest "maw" and throw as a weapon. Gonomes appear only in the expansion pack Half-Life: Opposing Force, which was created by Gearbox rather than Valve.
Fast headcrab zombie
When a fast headcrab attaches to a host, it produces a fast headcrab zombie. These zombies are stripped of their outer skin, some body tissue, and most or all organs, leaving large parts of the victim's skeleton exposed. Fast headcrab zombies can run at great speeds and have the unique ability to jump and rapidly climb rain gutters. They also move on all four limbs, but stand on two legs when attacking a victim. Fast zombies betray their presence with several vocalizations. In Half-Life 2: Episode Two, the fast zombie can be seen jumping on the front of the players vehicle, and melee attacking until Alyx Vance kicks it off. Despite their speed and physique, fast zombies are much weaker than ordinary zombies in terms of damage they inflict, though they have the same health as standard zombies. Though they are not as common as standard zombies, they often attack in groups of two to three and can be difficult to hit due to their speed. In Episode Two, fast zombies also come in a torso-only variant.
Poison headcrab zombie
Poison headcrabs produce the poison headcrab zombie, a bloated, reddish/purplish, slow-moving and hunched-over "carrier" for several poison headcrabs. A fully-loaded zombie carries four headcrabs including the host's controller. It is the strongest and most resilient form of headcrab zombie in terms of damage capability and overall health, but also the slowest. A poison zombie reveals its presence with its distinctive, strangled, heavy breathing and muffled moans. Like other zombies, it tries to beat its prey to death, but its most dangerous weapon is the extra poison headcrabs it carries, which it can throw at the player with an elephant-like bellow.
Half-Life 2: Episode One introduces the Zombine, a variation on the regular headcrab zombie created from transhuman Combine soldiers. Alyx Vance coins the term "Zombine" for them as a portmanteau of "zombie" and "Combine". The introduction of the Zombine enemy was meant to indicate that, in the wake of the devastation caused by the player in Half-Life 2, what was once a valuable weapon in the Combine arsenal is now just as dangerous to them as humans.
A zombine can be identified by the distinct Combine armor it wears, as well as its electronically filtered moans occasionally mixed with Combine soldier reports, such as "Necrotics inbound" or "Sector is. . . not secure." The body armor of the Combine soldier hides the chest and mouth. Removal of a zombine's headcrab reveals that most of its head has been removed, leaving only the lower jaw and a small lower portion of the skull. The official Prima strategy guide for Episode One indicates that the lack of a head is meant to be an intentional mystery. Zombines also lack the elongated claws of their counterparts, though, unlike their standard cousins, have the ability to produce grenades which can be launched at the player.
Introduced in Half-Life 2, Lamarr is a headcrab belonging to Doctor Isaac Kleiner. Having been debeaked, thereby eliminating her ability to attach to and control a host, Lamarr has been rendered into a semi-tame pet for Kleiner and a nuisance to anyone else with whom she comes into contact. Kleiner states, "The worst she might do is attempt to couple with your head…fruitlessly," behavior that leads Barney Calhoun to say, "Freakin' head-humper." Lamarr is named after the late 1930s actress and inventor Hedy Lamarr, as evidenced when Dr. Kleiner alternatively calls her Hedy in the conclusion of the chapters "Red-Letter Day" and "Entanglement."
Lamarr serves as the catalyst for the first few chapters of the single-player campaign in Half-Life 2. When Gordon Freeman attempts to use Kleiner's teleporter, Lamarr jumps from a vent and damages it while Gordon's teleport is in progress, which forces Gordon to proceed on foot once the malfunction subsides. When Alyx and Gordon return to the lab some time later, Lamarr escapes her cage, and Alyx has to stay behind to help Kleiner catch her, as Kleiner refuses to leave without her. Lamarr makes her final appearance in Half-Life 2 after the credits, dropping down into a black background and then leaping toward the screen as part of a stinger sequence. Lamarr is safe and sound in Episode One, where she makes a brief cameo appearance, causing further mischief to Dr. Kleiner during his unedited evacuation broadcast in City 17. She makes yet another appearance in Episode Two, where the player can see her climb into Dr. Magnusson's rocket, which is launched shortly before the end of the game. Seconds before the launch, Dr. Kleiner notes a payload anomaly of approximately 8.5 pounds, suggesting that Lamarr has been launched into space.
Use as biological weapons
While headcrabs are merely wild parasites in Half-Life, Half-Life 2 shows that the Earth-occupying Combine have put them to use as an unstable but effective biological weapon against the human Resistance. A coffin-like missile is filled with headcrabs, and fired from a mortar (as seen in Half-Life 2: Lost Coast) The Combine will bombard distant areas with these missiles and the payload of each shell is released, free to infest or kill nearby victims without risk to the Combine forces. As is the unfortunate case of the devastated town Ravenholm, repeated bombings can neutralize entire towns and cities in a short span of time.
Headcrab shells commonly carry standard headcrabs and fast headcrabs, as can be seen throughout Half-Life 2; poison headcrabs are only seen emerging from headcrab shells during the church battle in Half-Life 2: Lost Coast and in the Exit 17 chapter of Half-Life 2: Episode One. The only two examples in Half-Life 2 are where the player can enter a house which has been bombed by a headcrab shell during the Highway 17 mission (Searching the house will reveal several poison headcrabs and a poison headcrab zombie, but no other types, indicating that the shell contained poison headcrabs) and in the Water Hazard level, where the player can find multiple pods lying in a fenced area. Nearby, there are headcrab zombies and dead rebels, suggesting that the headcrabs are dangerous and giving the player an idea of what they do. The number of headcrabs in a missile varies between species of headcrab.
The poisonous head crab was ranked the second most terrifying video game enemy of all time by Cracked, due to their venom's ability to drain the players' health to one.
Due to popular request, Valve released a plush headcrab for sale at the Valve Store. It featured posable limbs, a number of teeth and claws and a gaping maw. The headcrab went out of sale within a few months. For a 2006 Christmas special, along with the re-release of the plush headcrab, fans could buy a headcrab hat, specially designed to give the impression that the wearer is under attack from the parasitic alien.
- The description for the plush headcrab collectible item indicates that the six-inch toy is quarter-scale, meaning that a normal-sized headcrab is typically two feet in length.
- Half-Life 2: Episode One, Chapter III: Lowlife, Developers commentary 
- HL2 Headcrab Collectible. Retrieved on 2009-02-18
- HL2 Headcrab Hat. Valve Store. Retrieved on 2009-02-18