Prosternum

Definition

The ventral (sternal) sclerite of the prothorax, the foremost thoracic segment of an insect. Together with the pronotum (dorsal) and the propleura (lateral), the prosternum forms the exoskeletal box of the prothorax, which bears the forelegs but never wings in extant insects. Its shape and armature—such as the presence of a prosternal process, spines, or grooves—are frequently diagnostic in beetle families (e.g., Elateridae, where the prosternal spine fits into a mesosternal groove to form the clicking mechanism).

Etymology

New Latin, from Greek pro- (before, foremost) + sternon (chest, breastbone)

Example

In click beetles (Elateridae), the prosternum bears a posteriorly projecting spine that engages with a matching cavity on the mesosternum; when the beetle flexes its body, the spine slips past the catch, producing the audible 'click' that rights the insect.

Related Terms

• prothorax
• pronotum
• Propleuron
• mesosternum
• metasternum
• sternum
• Sclerite

Usage Notes

Distinguished from the mesosternum and metasternum of the middle and hind thoracic segments, respectively. In descriptive taxonomy, prosternal characters (shape, width relative to procoxae, presence of processes or pits) are routinely used in keys for Coleoptera and other groups. The term is occasionally misspelled or conflated with 'prosternal' (the adjectival form). Do not confuse with the genus Prosternum Gistel, 1856 in Elateridae, which is a separate taxonomic usage.