The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax. This fusion of the head and thorax is seen in chelicerates and crustaceans; in other groups, such as the Hexapoda, the head remains free of the thorax. An exception is females in Stylopidia, a clade in the order Strepsiptera, a group of highly specialised parasitic insects. In horseshoe crabs and many crustaceans, a hard shell called the carapace covers the cephalothorax.