Headcrab

Headcrabs are cat-sized, mite-like creatures which come from Xen, though most were teleported to Earth in large numbers. 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. If the host is still partially alive and half aware to what has happened to him, less it be combine, played backwards the standard headcrab zombie's screams of pain are interpreted correctly as him calling for help.

Standard Headcrab
The most common headcrab to be located is the "ordinary" headcrab. This 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. As per nearly all "most common subspecies", 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.

Fast Headcrab
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 Headcrab
Poison headcrabs are stronger than the other headcrabs. They have black skin and many hairs 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 that is similar to a rattlesnake rattling it's tail.

Armored Headcrab
The Armored Headcrab is a spiked variant of the normal headcrab. Its shell is visibly much bumpier, and has a grey color. It's spiky armored shell seemingly comes from an unknown Xenian species of plant or fungus life that can be seen throughout the environment. Due to this shell, Armoured Headcrabs are highly resistant to damage and can only be killed through damage to their underside.

Reviver Headcrab
A sub-species of Headrab which seems to share a symbiotic relationship with others, the Reviver will not seek out and take control of a host. Rather, it will latch onto a dead body of a former Headcrab host and reanimate them. The Reviver will not strictly attach to the corpse's head, but will usually go for the torso instead. Strangely, Revivers also possess a bioelectric attack, enabling it to fire bolts of electricity at threats. When attached to a corpse, this electric attack is enhanced into a more wave-like burst.

Gonarch
Considered the final phase of the Headcrab lifecycle, the Gonarch serves to breed new Headcrabs. It resembles a giant quadrupedal creature coated in a dark-brown carapace, complete with an eggsack hanging from the central mass. When threatened, the Gonarch will attack using its pointed legs or by spraying acid from an opening on its upper side. It will also constantly spawn baby Headcrabs to aid it. The Gonarch is completely impervious to damage unless directed at the eggsack.

Life Cycle
Whilst the life cycle largely remaines unknown meant theory's have been put forward with some believing the infection process as unnecessary whereas others believe the zombie to mutate into a gonarch or that the zombie is the result of failed infection with the hole in there chest being exposure of an organ ( only found in the original prey ) which would later become the birthing sac, it has also been speculated that the headcrabs require mating with each other before coupling with one's head for the host/zombie to become a gonome (humanity's equivalent of a gonarch ) or a gonarch later in life which will then produce offspring.

Appearances

 * Half-Life (1998)
 * Half-Life 2 (2004)
 * Half-Life: Alyx (2020)