prosternum

Prosternum is a ventral sclerite of the prothorax, the first segment of the thorax, in many arthropods such as insects and crustaceans. It forms part of the thoracic exoskeleton on the underside and typically lies between the coxae of the first pair of legs, contributing to the floor of the thorax. The exact form of the prosternum varies across groups: in some species it is a single plate, in others it is subdivided or reduced, and many insects possess a distinct prosternal process that projects forward or downward.

Function and variation: The prosternum provides attachment sites for muscles that move the first legs and, together with surrounding sternites, helps stabilize the thoracic confinement. Its morphology can influence how the thorax articulates with other body regions in different taxa. Developmentally, the prosternum is part of the ventral thoracic exoskeleton formed during molting and metamorphosis in many species.

In vertebrate anatomy: The term prosternum is rarely used. In humans and other mammals, the anterior chest bones are described as the sternum, with regions such as the manubrium, body, and xiphoid process, rather than a prosternum. Some older or less common sources may employ the term, but it is not part of standard human nomenclature.

Etymology: from Greek pro- “before” and sternon “breastbone.”