pygidia

The pygidium is the posterior-most part of an arthropod's body, formed by the fusion of abdominal segments. It is typically found at the very end of the abdomen. The shape and size of the pygidium can vary greatly among different arthropod groups, and it often serves important functions.

In crustaceans, such as trilobites (which are now extinct), the pygidium was a prominent feature. In many insects, the pygidium is less distinct but can be involved in reproduction or respiration. In arachnids, the pygidium is greatly reduced or absent.

The pygidium can have various appendages or structures, depending on the species. For instance, it may bear spines, cerci (paired appendages at the end of the abdomen), or a telson, which is a terminal segment that may bear stinging or sensory organs. The morphology of the pygidium is often a key characteristic used by scientists to classify and identify different arthropod species. Its presence, absence, and specific form can provide valuable clues about an arthropod's evolutionary history and ecological niche.