Comparison of the External Morphology of the Sternal Glands for Hornets in the Genus Vespa .

Many social wasps in the speciose subfamilies Polistinae and Vespinae have two sternal glands-the van der Vecht gland and the Richards gland-that are not found in other insects. The presence of these glands has been confirmed in only 6 of 22 hornet species (genus Vespa ) and images of their fine structure have not been produced. Here; we characterize the external morphology associated with both glands for workers of nine Vespa species using scanning electron microscopy. All hornets had similar gland configurations; although gland-associated external features differed among species. Scaled for size, glands were equivalently sized for the giant hornets ( V. mandarinia and V. soror ) and their closest phylogenetic relatives ( V. tropica and V. ducalis ). Relative size of gland-associated structures was reduced by half for V. simillima ; V. velutina ; and V. affinis workers. The remaining species ( V. crabro and V. analis ) had intermediately sized features. Differences among species in external gland structure were best explained by selective pressures related to predatory behavior, rather than defense of nests against ants. However, a lack of information about how Vespa workers use their van der Vecht and Richards glands limits a comparative interpretation of the function of their external gland morphology.