Headcrab

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. Headcrabs were studied at Black Mesa, notably in the Advanced Biological Research Lab and the Gamma Labs. A Headcrab's attack does little damage, and it also takes some time for Headcrabs to take over their hosts, who are initially still in control; however, Headcrabs both attack very swiftly and (should they succeed) latch onto the head very firmly, so that any attempt by a struggling victim to remove the parasite with their hands is largely futile. Coupled with disorientation and the imminent loss of consciousness from suffocation, this ensures that most victims nonetheless succumb very quickly, explaining the exceptional effectiveness of the Headcrabs' use by the Combine. It is not known for sure if a victim of a Headcrab attack can still be rescued once a Headcrab has attached itself onto their head