Pygidium

: pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites.

[1] In arachnids, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites.

A pygidium is present in Palpigradi, Amblypygi, Uropygi, Schizomida, Ricinulei and in the extinct order Trigonotarbida.

Pygidia that are similar in shape and form to the thoracic segments are termed homonomous, while pygidia that vary significantly from the shape and form of the thoracic segments (like by the presence or absence of spines) are heteronomous.

In insects, the pygidium is the dorsal tergite of the last external abdominal segment.