The middle portion, or body, of the sternum.

The ventral piece of the middle segment of the thorax in insects.

Synonymous with mesothoracic venter, that is, used for the ventral portion of the mesopleuroste rnum below the transepisternal line/sulcus, if such a line is visible. The ancestral mesosternum is hypothesized to be invaginated into the thorax and not externally visible in Hymenoptera, though a mesosternum has erroneously been ascribed to some Encyrtidae and Eupelmidae by some authors.