Headcrabs are parasitic life-forms 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 or 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: while headcrabs in Half-Life can swim but die in toxic environments, 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 towards toxic and radioactive materials. The games also establish that while headcrabs are parasites that prey on humans, they are also prey to 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.
The Fast Headcrab looks much slimmer and more agile than a normal Headcrab, having thinner, almost spider-like legs. They also have slightly less bulky bodies and, surprisingly, do not have a "beak" like other Headcrab variants are known to have. Fast Headcrabs share many of the vocalisations of the standard Headcrab. Examined up close, Fast Headcrabs have wart-like bumps on their skin in certain areas. The legs of a Fast Headcrab also have elongated claws, which as well as serving as weapons and a means to latch on to surfaces, waiting to attack unsuspecting prey. This is seen in Half-Life 2: Episode One, Lowlife. They drop off when the player gets too close or shines their flashlight on them. Fast Headcrabs have been seen cooked and eaten by Vortigaunts which suggests that they are somewhat palatable as a food source. Fast Headcrabs are more commonly seen eaten than standard Headcrabs, which probably indicates they taste better than standard Headcrabs.
The Poison Headcrab can be identified by its dark colored skin (sometimes with a wet sheen) and thick hairs on the joints of its inward bending, spider-like legs. On the Poison Headcrab's legs are white bands that encircle the knee joints and the creature's dorsal markings are similar to those of a species of orb-weaving spider