pygidion

Pygidion is a term used in biology, primarily in entomology and comparative anatomy, to refer to the posterior-most part of an arthropod's body, formed by the fusion of the last few abdominal segments. It is distinct from the telson, which is the terminal appendage of the body, though in some species, the pygidium may incorporate or be fused with the telson.

The pygidium is typically a hardened, often flattened or shield-like structure. Its appearance and size vary greatly among different arthropod groups. In crustaceans, for example, the pygidium is often referred to as the tail fan. In insects, the pygidium is usually less prominent and is often part of the terminal abdominal segments, which may bear cerci or other appendages.

Functionally, the pygidium can serve various purposes. It may provide protection for the delicate internal organs of the abdomen, act as a lever for propulsion (as in some crustaceans), or be involved in reproductive structures. In fossil arthropods, the pygidium can be an important feature for classification and understanding evolutionary relationships. The presence and morphology of the pygidium are key characteristics used in identifying and differentiating various arthropod taxa.