Tagmosis is the evolutionary process of fuse and modifying segments in metameric organisms such as arthropods to form tagmata. In insects, the body segments (somites or metameres) have become fused into three main tagmata: the head, thorax and abdomen. Segments making up each tagma can be either completely fused (e.g. the head capsule) or more loosely connected (e.g. the abdomen).

The Arthropod body plan has undergone various forms of regional specialisation or tagmosis to produce segment groups specialised for different functions. As previously stated these specialised regions are called tagmata. Tagmosis is an extreme form of heteronomy, mediated by Hox genes and the other developmental genes they influence.

Tagmosis varies among the arthropod groups:

Trilobitomorpha- an extinct arthropodic group contains three tagmata- the cephalon (head), thorax and pygidium

Crustacea- also contain three tagmata- the head (cephalon), thorax and abdomen

Hexapoda- also contain three tagmata- the head, thorax and abdomen

Myriapoda- contain 2 tagmata- cephalon and trunk

Cheliformes- contain 2 tagmata- prosoma and opisthosoma