The metapostnotum is present and distinguishable in members of nearly all examined families of Hymenoptera; its anatomy and associated structures are described and figured for representatives of nonaculeate families. On the basis of the evidence from the metapostnotrum and associated structures the Orussidae are the extent sister-group of the apocritan Hymenoptera, and the Cimbicidae are convergent with respect to metapostnotal structure with the Orussidae and Apocrita. Parallel reductions in the metapostnotum and associated muscles have occurred in the Ichneumonoidea, Chalcidoidea, Proctotrupoidea, Cynipoidea, and Ceraphronoidea; in all of these groups, at lest some species have relatively unreduced structures.