|Average height||0.5-1 ft. (subspecies vary)|
|Danger rating||Extreme threat|
|Subspecies||Fast, Poison, Gonarch|
Headcrabs, often called "head humpers", are the most numerous species from the alternate dimension known as Xen.
Headcrabs are cat-sized, crab-like (or perhaps tick-like) creatures. They come from Xen, but there are some subspecies that are not indigenous to that region; rather, most of these subspecies live on Earth. All headcrabs have the ability to infect hosts by opening the host's head and driving its spinal cord through the victim's head, connecting it to the victim's spine, which allows the headcrab to control the victim. The host may remain conscious after this process, as they constantly moan or scream.
The most common headcrab variant has pale tan skin and dark red legs. When threatened it will lift up its front legs in a way reminiscent of a tarantula. It attacks by leaping at an enemy's head and biting with its beak-like jaws. This group is also the weakest subspecies of headcrab, and a whack with a blunt object will usually bring it down. It walks around using its teeth and when it stands still, it will use its talon like front legs, which are also used for propelling it forward during an attack. Headcrabs are used as a biological weapon by the Combine.
As their name implies, these headcrabs have longer legs, increasing their speed and making them difficult to hit at close range. This headcrab variety also employ better tactics, weaving around their opponent to strike from behind. Fast headcrabs have never been encountered on Xen, only on Earth, suggesting they may have been created by the Combine purely for use as biological weaponry. They seem to possess the ability to cling to walls.
Poison headcrabs are stronger then the other headcrabs. They have black skin and many small spikes on their body, heavily resembling a four legged spider. Instead of biting and damaging their opponents, these headcrabs deliver a potent neurotoxin to their victims, making them easy prey for other enemies. They are very slow, more so than even ordinary headcrabs. Like the fast headcrab, they`re a new subspecies, complimenting their swift brethren nicely. It emits a very distinctive chittering noise.
Little is known about how and when the metamorphosis from a headcrab to a Gonarch takes place, as few ever reach that stage. While appearing very similar to a headcrab's anatomy there are a few differences; namely their massive size (twice as tall as a human), thick exoskeletons, powerful legs and the large sac dangling from its underbelly. Gonarchs are easily one of the largest and toughest known creatures from Xen and can take massive amounts of projectile and explosive damage.
During battle, Gonarchs can release swarms of baby headcrabs to attack their foes. Gonarchs also cry out whenever one is killed, suggesting a maternal bond. It is theorized that they sense vibration and scent, as they display no sense organs. Gonarchs also have an apparent lack of a "head" structure, suggesting that any brain may be burrowed deep within the armored exoskeleton for further protection. While no Gonarch has been observed ingesting any material, it does not appear to have a mouth of any sort, which brings into question its diet and method of eating. But it might be that the headcrab eats a lot before reaching the Gonarch stage, storing the nutrients to reproduce and feed its offspring until it eventually dies of starvation.
Once a headcrab gets a firm grip on a victim's head, it will infect and take over the new host's nervous system, turning it into a headcrab zombie. The type of headcrab determines the type of zombie, each having different abilities. The ordinary zombies use elongated claws to strike at their victim, their halfway-rotting corpse is split open and is proven to be a mouth as it was shown to be revealed within the Black Mesa Research Facility.
The Gonome is evolved from the standard zombie. It can toss gobs of explosive slime at its opponents, and has grown longer claws. It sits somewhere between normal and fast zombies in terms of speed.
To increase their speed and mobility, fast zombies have done away with their skin and organs. They are very fast, bounding on all fours to their target, and can easily jump 2 meters (6 and a half feet). With these abilities, they are obviously more deadly then the ordinary zombies. This is an example of a headcrab that has more control over the host.
Poison zombies are the strongest of all zombies. They can carry up to four other poison headcrabs on their back, which they throw at potential hosts. They can withstand many gunshots, but they are very slow.