Tagma (biology)

: tagmata – τάγματα - body of soldiers; battalion) is a specialized grouping of multiple segments or metameres into a coherently functional morphological unit.

Usually the term is taken to refer to tagmata in the morphology of members of the phylum Arthropoda, but it applies equally validly in other phyla, such as the Chordata.

For example, the one-time terms "cephalothorax" and "abdomen" of the Araneae, though not yet strictly regarded as invalid, are giving way to prosoma and opisthosoma.

In the ancestral arthropod, the body was made up of repeated segments, each with similar internal organs and appendages.

[2] The first and simplest stage was a division into two tagmata: an anterior "head" (cephalon) and a posterior "trunk".

The trunk bore the appendages responsible for locomotion and respiration (gills in aquatic species).

In some groups, such as arachnids, the cephalon (head) and thorax are hardly distinct externally and form a single tagma, the "cephalothorax" or "prosoma".