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.