Headcrabs are small light yellow domes with four legs. Their two front legs are long grasping claws, and the two rear are simply stubs to walk on and jump with. Headcrabs find a human-like host and leap for the head, hence the name. They orient themselves on the head and cover the host's face. Their mouthlike structures somehow take control of their host's brain activities and keep them alive as the headcrab. The "zombie" then conforms over times to include long pointy claws on the hands and a mouth that opens between the ribs.
A fully grown headcrab turns into a gonarch, commonly called Big Momma. Four long armored legs and a circular shell to cover the top, gonarch is radial in symmetry. She has a large eggsack and gives birth to baby headcrabs often. Few headcrabs live to this age.