Summary::A headcrab is a predatorial Xenotheric species, whose natural hatching grounds lie within the vast plateaus of Xen. They are however extremely able and adapt to other environs easily, having been injected into Earth's ecosystem with startling resistance to Earth pathogens, and impervious to radiation or toxic wastes. A headcrab is a small, quadrupedal creature that does not possess a body; its four legs are attached directly to its head and are positioned to allow quick jumps while impairing general movement. It is about 18 to 30 inches in size and weighs 4 to 10 pounds.