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 'fast headcrab', for lack of a better description, 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. Unlike other headcrabs however, these fast moving variants can easily match a human for speed and attack with frightening speed. It has a smaller head, at about 14 to 20 inches in size, but possesses longer, more spindly legs and weighs only 3 to 5 pounds.