A section of the body of an arthropod that is formed by the fusion of mesodermal somites and has a distinct function and structure. The basic tagmata are the head, thorax, and abdomen, but the form of the tagmata (known as tagmosis) varies between arthropod groups, each group having its characteristic tagmosis. For example, many crustaceans have a cephalothorax and abdomen, while arachnids have a prosoma and opisthosoma.