Morphological variation in the gastral sterna of female Apoidea (Insecta : Hymenoptera)

A large amount of morphological variation is described for gastral sterna 2 through 6 of female bees and wasps belonging to the Apoidea. This variation includes differences among taxa in the degree of development of the antecosta, acrosternum, gradulus, sternal apodemes, and posterolateral areas and the size and shape of the postgradular region. There is considerable variation among sterna 2 through 6 within individual bees. Some of this variation is mostly consistent among taxa, such as the increasing anterior divergence of the sternal apodemes and the decreasing ratio of posterior to anterior width of the more posterior sterna. Other characteristics vary inconsistently among sterna within or among taxa. Examples include the presence or absence of graduli or premarginal lines on different sterna. Variation is most extreme for the apical external sternum, the sixth, which exhibits a large number of apomorphic traits, particularly in the Andrenidae and cleptoparasitic Apidae. The potential utility of sternal characters for phylogenetic studies of the Apoidea is discussed. There would seem to be particularly useful variation in the form of sternum 6 among different lineages of cleptoparasitic Apidae, presumably associated with different means of gaining entry to and (or) ovipositing within cells of the host bee. Sternal characters might help resolve relationships among these morphologically otherwise often strongly convergent taxa.