tagmata

Tagma (plural tagmata) refers to a recognizable unit of fused body segments in arthropod anatomy, functioning as a modular unit that carries out coordinated tasks. The term derives from Greek tagma, meaning “that which is arranged” or “a division.” Tagmosis, the organization of segments into tagmata, is a major aspect of arthropod body plan diversification and supports specialization of regions for different functions.

In insects, tagmosis typically yields three main tagmata: the head, the thorax, and the abdomen. The head houses sensory organs and mouthparts; the thorax bears legs and, in winged species, wings; the abdomen contains most of the digestive and reproductive organs. Variation exists among insects, but three-part division is a common, defining feature of hexapod form.

In chelicerates, such as spiders, scorpions, and horseshoe crabs, the body is divided into two tagmata: the anterior prosoma (often called the cephalothorax) and the posterior opisthosoma (abdomen). The prosoma carries the mouthparts, eyes, and legs, while the opisthosoma contains the viscera and reproductive organs; in spiders, spinnerets are located on the posterior tagma.

In crustaceans, tagmosis often produces a cephalothorax (a fused head and thorax) and a distinct abdomen (pleon), though some groups retain separate head, thorax, and abdomen without fusion. The cephalothorax typically bears feeding appendages and first limbs, while the abdomen bears pleopods and tail structures in many species.

Tagmosis has clear evolutionary and functional significance, enabling specialization of limbs and organs and contributing to the wide morphological diversity observed across arthropod groups.