headthoraxabdomen

Headthoraxabdomen is a term used in comparative anatomy to refer to the three main body regions, or tagmata, that organize the body plan of many arthropods. In most insects and many crustaceans, the body is divided into a forward head, a middle thorax, and a posterior abdomen. These regions reflect functional specialization and a modular pattern of development known as tagmosis. Not all species use the same arrangement; in crustaceans the head and thorax may be fused into a cephalothorax or prosoma, while in arachnids the body is described as a cephalothorax (prosoma) and an abdomen (opisthosoma).

The head houses sensory structures and mouthparts; in insects it typically bears the antennae, compound eyes, ocelli, and complex mouthparts such as mandibles and maxillae. The thorax primarily provides locomotion; it bears legs and, in winged insects, the wings on the mesothorax and metathorax. The abdomen contains much of the digestive, excretory, and reproductive systems and bears posterior spiracles in many groups, with segmentation that can be reduced or specialized in particular taxa.

In development and taxonomy, the relative size, segmentation, and fusion of these regions help distinguish groups. The term emphasizes the anterior-posterior organization rather than a single continuous structure. Across arthropods, variations include fused cephalothoraxes in crustaceans and the distinction between prosoma and opisthosoma in arachnids, illustrating the diversity of headthoraxabdomen configurations.