Cephalothorax

The cephalothorax (prosoma) is a fused head–thorax body region found in several arthropod groups; it bears sensory organs and limbs and is often protected by a carapace.

The cephalothorax, also called the prosoma, is a body region in which the head and thorax are merged into a single unit. This tagma appears in a number of arthropods and serves as the attachment site for many of the animal's principal sensory organs and locomotor appendages. The term emphasizes the functional fusion of two separate body regions into one structural module.

Structure and typical components

The cephalothorax usually contains the brain and major ganglia, mouthparts, eyes and the base segments of limbs. Specific features vary by group, but common elements include:

anterior sensory structures such as compound or simple eyes and, in some groups, antennae;
feeding appendages (mandibles, maxillae, chelicerae or similar structures);
walking legs or specialized limbs used for feeding, sensing or reproduction.

In many species the dorsal surface of the cephalothorax is covered by a hardened shield called a carapace, formed from the exoskeleton to protect soft tissues and streamline the body.

Distribution and evolutionary context

The cephalothorax is common in major arthropod lineages such as crustaceans and chelicerates. It contrasts with insects, which retain a distinct head and thorax. The fusion of head and thorax is interpreted as an evolutionary modification that concentrates sensory and feeding structures and provides a compact platform for limb articulation.

Functionally, the cephalothorax centralizes control of feeding and locomotion and often plays a defensive role when reinforced by a carapace. Notable examples include crabs and lobsters among crustaceans and spiders, scorpions and horseshoe crabs among chelicerates. For a broad definition of animals in which such tagmata occur, see arthropods and discussions of the relation between the head and thorax in comparative anatomy.

Recognizing the cephalothorax helps distinguish major body plans: it is a defining tagma for several groups, differs from a simple head, and highlights how segmentation can be modified to meet ecological and functional demands.