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 poison headcrab is a fairly large variant of the headcrab, but shares the same physiology type, bodiless, with powerful and long legs attached directly to the head. The poison headcrab is easily recognized by its much larger size and its black, mottled body. Excluding the long legs, its head is around 20 to 26 inches in size.