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Headcrab

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File:Headcrab.jpg
Left: A headcrab from Half-Life. Right: Its appearance in Half-Life 2.

A headcrab is a fictional alien species found in the Half-Life and Half-Life 2 computer game series created by Valve Software. Headcrabs are also referred to as head-humpers by Barney Calhoun and parasitics by the Combine Overwatch. They are the most numerous aliens in the series, and have the ability to attach to humans and transhumans to form zombies.

Physical description and behavior

File:HalfLife WalkingHeadcrab.png
While walking, a headcrab can be seen lifting its larger claws upward, leaving two of its stubby legs to provide frontal support. In Half-Life 2, the motion of its legs is more prevalent, suggesting that they are, in fact, part of the headcrab's walking mechanism.

The headcrab is a small creature with a design consisting of a small rounded body with four legs for movement (two stubby legs at the front, obscured by its claws, and two hind legs at the back) and a pair of larger frontal claws for attacking prey or as additional support when standing still. Under its body is a large rounded mouth filled with sharp teeth. Headcrabs have a mottled tan-coloured skin and reddish legs (or blood). The headcrabs found in Half-Life are noticably smaller than those in Half-Life 2 which have bodies approximately the size of a watermelon. A Valve site selling plush headcrabs [1] describe 6" as being quarter-scale, implying that headcrabs are typically 2 feet in length. Curiously, although Half-Life 2's headcrabs are capable of tolerating toxic waste and radioactivity, they are unable to survive in water; these traits are completely inversed 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.

Although small, slow-moving, and relatively weak on its own, 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 body.

Headcrabs are known to bury themselves in the ground to hide, undetected, then climb out to the surface and attack when a victim draws near. This surprise tactic is employed similarly by Antlions. Headcrabs also have a penchant to lie in wait in dark corners and ventilation shafts. At the Black Mesa Research Facility, headcrabs are often encountered while crawling through air ducts.

Headcrabs can produce a variety of vocalisations. When they are not hunting, headcrabs usually emit squeaks and fairly quiet, repetitive calls, whilst gently swaying their bodies back and forth. When attacking, the headcrab will emit a sharp, shrill shriek as it leaps towards its victim.

File:HalfLife BabyHeadcrab.jpg
An underdeveloped infant headcrab, as seen from Gonarch's lair in Xen, Half-Life.

Headcrabs are often seen clustered in groups. They will pursue their prey under the most dangerous conditions, moving through dangerous areas or towards hostile prey with little concern for the consequences, even as their fellow headcrabs die in front of them.

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

Interestingly, no Xen species are seen infested with headcrabs. Indeed, the physical makeup of the headcrab, and its apparent need to clamp on the 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. Lamarr, Dr. Kleiner's pet headcrab, is fed watermelons, a fact noted on a Post-it note in his laboratory, although she is also seen to attack a crow later on in the scene. On the other hand, headcrabs have been observed to be prey by bullsquids, vortigaunts, and barnacles.

Half-Life 2 headcrab variations

File:HalfLife2 FastHeadcrab.jpg
A fast headcrab.

New to Half-Life 2 are two previously unseen varieties of headcrabs. It remains to be seen if they are native to Xen (as they never appeared in Half-Life) or are result of genetic manipulation of the original form by the Combine.

The first variation is known as a fast headcrab, having longer, thinner, almost spider-like legs, which allows it move much faster. It does not have a "beak" like the other varieties, it seems to use the sharp talons at the ends of its legs to latch to hosts instead. It is also able to cling to walls and ceilings.

The second variation, a poison headcrab, is identified by its dark sage green color skin (sometimes with a wet shine) and thick hairs on the joints of its inward bending legs. To distinguish it further it has white bands that encircle its knee joints. It also makes a distinctly sinister chirp at range (occasionally described as cute) and a hissing/rattling similar to that of a rattlesnake's tail when it detects a viable host. According to Half Life 2: Raising The Bar, the development team noticed that upon hearing the poison headcrab's sound playtesters would frantically start looking around for the poison headcrab so they could destroy it, regardless of any other present dangers. Poison headcrabs move slowly and cautiously when maneuvering but will leap with incredible speed and height with an angry squeal when it has a clear line of sight to the player, and run around frantically when shot at. To further add to their fearsome nature, the dorsal markings of the Poison Headcrab are modelled on a species of orb-weaving spider, Araneus diadematus [citation needed].

File:HalfLife2 PoisonHeadcrab.jpg
A poison headcrab.

A poison headcrab's neurotoxin is extremely powerful and hazardous to non-hosts; in gameplay it reduces the player's health to 1% immediately upon contact regardless of the current health amount (but health slowly recovers if they remain away from harm for a period of time, due to the HEV suit quickly providing an antidote). As the poison headcrab's only method of attack is by its poison and not from direct physical damage, there is no way for it to kill someone outright. Even a whole army of poison headcrabs continually attacking the player will not be able to kill them as the most that can happen is the reduction of one's health to 1%. However, this does make the player extremely vulnerable to enemies which are in the vicinity of poison headcrabs, as a player attacked by a poison headcrab is unable to resist even the slightest damage until they recover. Poison headcrab attacks will, however, reduce the player's shield levels.

While appearing similar to the original headcrab, both headcrab variations differ in a sense that they strictly use their four longer limbs for movement, attacking and standing, as opposed to six from the original headcrab.

Headcrab zombie

A headcrab's primary goal is to attach to the head of a suitable host using its mouth, 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 is vaguely similar to the facehugger in the Alien movies, although their purposes for their hosts are different. This similarity has led to some debate over whether the headcrab is a "rip-off" from the classic science-fiction/horror films or simply a homage to the films. 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 [2]. A similar plot device was also used in Ian Livingstone's fighting fantasy book Island of the Lizard King, involving a creature called a "Gonchong" whose proboscis would bury into the head of a host to control it.

File:Headcrab ZombieComparison.jpg
A headcrab and host from the original Half-Life (left) and Half-Life 2 (right).

When a headcrab successfully attaches to a host, the host becomes a "headcrab zombie", or "necrotic" 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. 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.

File:HalfLifeOpposingForce Gonome.jpg
A gonome in Opposing Force.

The gonome is apparently the next step of headcrab zombie "mutation". It is larger, stronger, and more intelligent than "ordinary" zombies; it also produces a volatile saliva (similar to that of a bullsquid) that the creature can retrieve from its chest "maw" and throw as a weapon. Gonomes can create lairs, or "nests", for themselves with enough time. Officially, gonomes appear only in the expansion pack Half-Life: Opposing Force.

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 damaged 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. In Half-Life 2, playing parts of a victim's sound file backwards produces extremely disturbing yells of pain. The reversed cries have been interpreted as "God help" and "help me". An example of this can be found here.[3]

Half-Life 2 variations

File:HalfLife2 FastZombie & PoisonZombie.jpg
A fast headcrab zombie (left) and a poison headcrab zombie with several additional poison headcrabs riding on the host's back (right).

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.

Fast headcrabs produce leaner zombies, stripped of their outer skin and some body tissue (revealing large parts of the victim's skeleton) along with the standard elongated claws. These zombies are astonishingly fast, agile, and have the ability to leap long distances and climb quickly. Fast zombies also move on all four limbs, but stand on two legs, as do the other types of headcrab zombies. Common nicknames include "Shrieker", "Howler" and "Leaper".

Poison headcrabs produce bloated, slow-moving hunched-over "carrier" zombies that seem to have even more endurance and resilience to damage than their brethren. Usually seen encumbered with several other poison headcrabs hitching a ride atop it, when this zombie senses an enemy, it hurls these hitch-hiking headcrabs with great distance and accuracy, although the headcrabs may jump off by themselves. Carrier zombies are not, themselves, poisonous; if one has no more headcrabs to hurl, it will attack with its claws like a traditional zombie. Their presence can be detected by their strangulated heavy breathing or muffled moans.

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 chest cavity that a normal headcrab zombie possesses, although fast zombies do have cracked rib cages and seemingly no internal organs, creating a hollow space.

Half-Life 2: Episode One variation (Zombine)

File:Zombine2.jpg
A headcrab-controlled Combine soldier.

Half-Life 2: Episode One depicts a new type of headcrab zombie - Combine Zombies - created as a result of headcrab infestation on a transhuman Combine soldier. It should be noted that only Zombines controlled by normal headcrabs appear in Episode One. Alyx coins the term "Zombine" as a portmanteau of "Zombie" and "Combine".

Zombines move faster than normal headcrab zombies, but are slower than the fast variety. They are stronger than their normal counterparts, (they take three normal shotgun shots to kill) likely due to their bloodied but intact battle armor and quasi-mechanical bodies. Zombines have the ability to use grenades, though unlike uninfected soldiers they will continue holding on to the active grenade, using it as a club until it explodes. This can be easily stopped by using the gravity gun to pull the grenade out of its hands and throw it away. Zombine groans are filtered through the soldier's radio.

The appearance of the Zombine is given as evidence that the Combine are losing control of City 17 following the events of Half-Life 2. Prior to the human uprising there were very few headcrabs in the city, and only in the more run-down areas. The combined effect of the damage dealt to the Citadel, the temporary defeat of Combine forces, and the use of Headcrab shells against rebel safehouses during the uprising portrayed in Half-Life 2 has clearly allowed them to roam more freely.

Removal of a Zombine's headcrab reveals that, aside from the lower jaw and small lower portion of skull, no head remains on the soldier's corpse. This leads to the question of how the Zombine is communicating through the radio system; while it would be assumed that a soldier's radio is located in its helmet, it could also be that it is somewhere else in the soldier's battle armor - indeed, it would fit with the Combine's "forced evolution" that it should be directly integrated with the larynx (voice box), as is evident in an image of a stripped Combine soldier, taken from the Half-Life 2 Model Viewer. The fact that the headcrab can control its host's headless body also means that it controls directly the host's nervous system, explaining why the headcrab zombies seem to keep so little of their former intelligence.

Another oddity is the absence of the soldier's helmet. It can be assumed that a soldier would never go into battle without a helmet, and while it seems possible that the headcrab's beak could puncture it, where the material itself went is a matter of speculation. Some possibilities include the helmet being discarded in pieces over time, or decaying during the zombification process in a similar fashion to the soldier's natural head, or that the helmet is no longer an armor add-on to the soldier, but part of the head itself. However, when the headcrab is removed, it is clearly visible that the entire skull has been removed, leaving the jaw attached to the neck.

A barnacle's response upon capture of a Zombine is also worthy of mention: While the body of an unaffected Overwatch soldier would be completely consumed by the creature, the barnacle will only consume the headcrab of the Zombine and discard the rest of the zombie's body.

Lamarr

File:HalfLife2 Lamaar & Kleiner.jpg
Doctor Isaac Kleiner attempts to coax Lamarr into hopping on his head in Half-Life 2 (due to Lamarr's "debeaking", the headcrab can no longer attach firmly to a victim to control it).

In Half-Life 2, the catalyst for the first few chapters of the single player campaign is the pet headcrab belonging to Doctor Isaac Kleiner, Lamarr, who wrecks Kleiner's teleporter (sending herself to a barren wasteland in the process) and so forces Gordon Freeman to travel on foot. Lamarr was named after the late 1930s actress and inventor Hedy Lamarr, as evidenced by a line issued by Dr. Kleiner just after Lamarr jumps onto the working teleporter during "A Red Letter Day" chapter, or at the end of the chapter "Entanglement". In the latter, as Gordon Freeman, Alyx Vance, and Dr. Kleiner are attempting to evacuate Dr. Kleiner's lab after it is discovered that the Combine are making an offensive sweep of the area, Dr. Kleiner states that he wishes to find and save his pet headcrab before they leave. Alyx attempts to rebutt him by telling him that there are many other headcrabs to use as a pet, and he replies by saying "There's only one Hedy." Lamarr is safe and sound in Episode One, where she makes a brief cameo appearance, causing further mischief to Dr. Kleiner during his live and unedited evacuation broadcast in City 17.

Lamarr is the first proof in the Half-Life series that it is possible to disable a headcrab's capability to couple and infest a host. Lamarr is said to be "debeaked" by cutting short a pair of its frontal "beaks", removing her ability to cut into a victim's head and assume control. Although Kleiner refers to Lamarr as a female, whether headcrabs actually have a gender remains unknown.

Combine application

File:HalfLife2 Headcrab-Shell.jpg
Headcrabs emerging from a landed headcrab shell, an offensive form of weaponry by the Combine.

In Half-Life 2, the Earth-occupying Combine alien race was seen utilizing headcrabs as a form of biological weaponry against the Resistance. "Headcrab shells", as they are known, are shells containing several headcrabs, fired from an unknown launcher (one such launcher, however, does make an appearance in Half-Life 2: Lost Coast, but it is unlikely that this is the only one), and its payload released into the open shortly after impact, free to infest or kill nearby victims. If released in large numbers, the shells are highly effective in neutralizing a large concentration of Resistance groups, as proven during a visit to the devastated town of Ravenholm. Standard headcrabs and fast headcrabs have been observed emerging from headcrab shells that have been launched, however poison headcrabs have only ever been seen emerging from unlaunched headcrab shells.

The Combine's mechanism for obtaining large numbers of headcrabs for use in these 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 [4]. Officially however, no reason has been given.