Tagma

Tagma, in biology, refers to a structural unit formed by the fusion of multiple body segments in arthropods, which then functions as a single, specialized region. The term comes from the Greek tagma, meaning order or arrangement. Tagmata contrast with individual segments, representing a higher level of organization through tagmosis, the developmental and evolutionary process that creates these functional modules.

In insects, the body is commonly organized into three tagmata: the head, the thorax, and the abdomen. Each tagma houses distinct structures and functions—sensory and feeding organs in the head, locomotor and sometimes wings in the thorax, and digestion and reproduction in the abdomen. In crustaceans, tagmata often include a cephalothorax (a fused head and thorax) and an abdomen, though arrangements vary among groups. In arachnids, the typical division is into a prosoma (or cephalothorax) and an opisthosoma (abdomen). Some myriapods and other arthropod groups exhibit different tagmata configurations, but the general principle remains: tagmata are functional modules formed by fused segments.

Tagmosis enables specialization of body regions, contributing to ecological diversity and complex locomotion. It is distinct from segmentation: a tagma is a functional unit comprising multiple segments, whereas a segment is a single repetitive unit. The study of tagmata and tagmosis sheds light on arthropod evolution, development, and the organization of the arthropod body plan.