Headcrab

"Have you ever seen such a magnificent species? These crabs can completely control their hosts' nervous system. Can you imagine what the next stage of mutation looks like?"

-An Black Mesa Scientist in OP4.

Headcrabs are Xen Creatures found in multiple HLC and HLRP games.

Info
''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.

Headcrabs were studied at Black Mesa, notably in the Advanced Biological Research Lab and the Gamma Labs.

During the Combine occupation of Earth, Headcrabs appear to have no external sensory organs, including eyes or ears, though the original Headcrab model in Half-Life had three eyes on each side of its body.

The Classic Headcrab can also be seen burrowing in and out of the ground, but not the other species.

Zombification
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 seem unlikely that they would evolve such abilities if there were 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 variation affects its host in its own way, producing a Zombie extremely distinct from the other types in both appearance and behavior.

Headcrab Zombies can mutate to Gonomes, which are bigger, more dangerous and faster zombies. Headcrabs lunge off the mutated gonome host to avoid further mutation, this is unknown why.

Types
There are not really alot of Headcrab variants in HLRP games, but in HLC there are multiple headcrabs.

Classic Headcrab
The classic headcrab, shown in BMI. These crabs can control a user's control and turn him into a zombie, then mutate to a gonome.

Xenocrab
A headcrab that can throw a lethal airborn poision in the air and has green particles all over the body. Shown in Damocles Sector.

Poison Headcrab
The Poison Headcrab (also commonly known as the black headcrab, named "Venom Crab" by Alyx Vance) is a Headcrab variation previously unknown before the Combine invasion. Like its relatives, the Poison Headcrab is able to infest and mutate humans into Zombies.

It is unknown whether Poison Headcrabs are a natural variation that's never encountered on Xen or a result of Combine genetic manipulation of the Classic Headcrab for use as weapons in Headcrab Shells. Shown in I17.