Pygidium

Definition

The posterior body shield or terminal segment complex of an arthropod, composed of fused tergites that cover the abdomen and protect the anus, genital opening, and—in females—the base of the ovipositor. In insects, the pygidium is typically the hardened dorsal plate of the last visible abdominal segment, often exposed when wings are folded. In trilobites and many crustaceans, it forms a distinct caudal shield separated from the thorax by an articulation.

Etymology

From Greek pygidion, diminutive of pugē (rump, buttocks)

Example

In female tephritid fruit flies, the pygidium is a conspicuous sclerotized plate at the abdominal tip that serves as a fulcrum during oviposition, allowing the telescoping ovipositor to penetrate fruit rinds; in trilobites, the relative size and segmentation of the pygidium versus the cephalon and thorax are key diagnostic characters for species identification.

Synonyms

• anal shield
• terminal tergum

Related Terms

• Tergum
• Abdomen
• ovipositor
• cercus
• Epiproct
• paraproct
• uropod
• Telson
• pleotelson

Usage Notes

The term is applied somewhat differently across arthropod groups. In insects, 'pygidium' usually refers specifically to the dorsal plate of the last visible abdominal segment (often the 7th or 8th), whereas the entire terminal segment complex may be called the pygidium in crustaceans and trilobites. Contrast with epiproct (dorsal unpaired terminal process) and paraprocts (lateral plates flanking the anus in some insects). Not to be confused with the taxonomic genus Pygidium (a group of catfishes in family Trichomycteridae).