As seen on Xen, Headcrabs appear to be typically born out of a Gonarch therefore, some Headcrabs are assumed to mature into Gonarchs as part of their natural life cycle, and may never use a host. While they do also posses what seems to be a set of four mandibles located where a normal mouth would be, they serve no obvious purpose, and so may simply be a vestigial evolutionary relic, akin to the human tailbone. Their underbelly is almost entirely covered by a large, jawless mouth, with a hidden 'beak' used for feeding and for latching onto the skulls of victims (see below).
These provide enough jumping power to leap up to 3 meters (10 feet) in any direction, albeit still requiring a direct line of visual contact between the Headcrab's eyes and its target before it can leap forwards, placing Headcrabs in a horrible position if the leap attack were to miss, requiring it to do a turn at a disadvantageously slow speed with its tiny legs. They are covered in smooth skin, which varies in color between subspecies, and walks on four short legs. Headcrabs are alien parasitoids, roughly the size of a pumpkin.
