cephalothoracic

Cephalothoracic is the adjective used to describe features associated with the cephalothorax—the fused head and thorax region in some arthropods.

Cephalothorax, from Greek kephalé meaning head and thorax meaning chest, is a body region formed by the fusion of the head and thorax in certain arthropods. It contrasts with the abdomen (opisthosoma) in organisms with a segmented trunk. The term is commonly used for spiders and other arachnids, as well as some crustaceans such as crabs and horseshoe crabs.

In spiders and many other arachnids, the cephalothorax (often called prosoma in spiders) is a single, hardened shield bearing the eyes and mouthparts, plus the eight legs, chelicerae, and pedipalps. It is typically connected to the abdomen by a narrow waist (pedicel in spiders), and is protected by a carapace. Sensory organs and feeding appendages arise from the cephalothorax, and respiration may occur via book lungs or tracheae.

In crustaceans with a fused head and thorax, the cephalothorax is usually covered by a single carapace and includes the first antennae and mouthparts, with walking legs attached to it. In many decapods (crabs, lobsters) the cephalothorax bears the major feeding appendages and is the main body mass, while the abdomen remains separate. In horseshoe crabs, the cephalothorax forms the dorsal shield that bears the eyes and six pairs of appendages, and it is followed by a segmented abdomen and book gills on the ventral side.

Not all arthropods have a distinct cephalothorax; in mites and ticks, the head and thorax are fused with the abdomen into an idiosoma, showing extreme tagmosis reduction. The concept of a cephalothorax highlights the modular organization of arthropod bodies and how evolutionary processes can merge segments to produce functionally integrated units.