In invertebrate biology, a tagma (plural tagmata) is a specialized grouping of arthropod segments, such as the head, the thorax, and the abdomen with a common function.[1] The segments of a tagma may be either fused or moveable.
Tagma divisions vary among taxa. For example, in trilobites the tagmata are the cephalon (head), thorax (body), and pygidium (tail), whereas in hexapods, these same divisions are called head, thorax, and abdomen. The body of spiders and some crustaceans is divided into two tagmata: the cephalothorax and the opisthosoma (spiders) or abdomen (crustaceans).
The evolutionary process that creates tagmata by fusing and modifying segments is called tagmosis, which is an extreme form of heteronomy, mediated by Hox genes and the other developmental genes they influence.[2]