Modifications of the metapostnotum and origin of the ‘propodeal triangle’ in Hymenoptera Aculeata

The metapostnotum of aculeate Hymenoptera shows three trends of modification: simple reduction in Bethyloidea, invagination and reduction in Vespoidea and enlargement (forming the ‘propodeal triangle’) in Sphecoidea (including bees). The second phragma is also variously modified in thete three groups to allow maximal development of the dorsal longitudinal indirect flight muscles of the mesothorax.