cephalothorax

Cephalothorax is a body region in some arthropods formed by the fusion of the head and thorax into a single anterior tagma. The term derives from Greek kephale (head) and thorax. In chelicerates, including spiders, scorpions, and related arachnids, the cephalothorax is usually called the prosoma. It bears the eyes, the mouthparts (chelicerae and, where present, pedipalps), and the main locomotory appendages (typically four pairs of legs). The abdomen, or opisthosoma, is separate and connected by a narrow pedicel in spiders. The cephalothorax often provides protection and houses sensory organs.

In crustaceans, the cephalothorax refers to the fused head and thorax, typically covered by a single carapace. This tagma includes the bases of the gills and feeding appendages, and it bears the major biramous limbs used for feeding and locomotion. In many species, the carapace extends forward as a rostrum and forms a shield over the body. Examples include crabs, lobsters, crayfish, and many shrimp. The abdomen usually remains distinct and may be flexible or segmented, depending on the species.

Insects generally do not have a cephalothorax; their head and thorax are separate tagmata joined only in function by the body plan, while the cephalothorax formation is characteristic of chelicerates and many crustaceans rather than insects.

The concept aids in describing body plans and evolutionary relationships among arthropods, and it highlights the specialization of the anterior segments for feeding, sensory input, and locomotion.