Comparative morphology of the mesosomal ‘gland’ in male large carpenter bees (Apidae: Xylocopini)

Abstract

The gross morphology of the reservoir of the mesosomal gland of male large carpenter bees (tribe Xylocopini) was examined in 46 taxa (species and subspecies) in the three genera of the tribe. Males of all but six species of Xylocopa examined possess a cuticle-lined invagination that originates along the suture between the metanotum and propodeum. Size of the reservoir and arrangement of the tubules are consistent within subgenera (with a single exception) but variable among most subgenera. The morphological extremes range from a simple, short invagination that opens medially to a complex set of convoluted tubules that open from paired, laterally-positioned openings and extend forward to the metanotum or scutellum and posteriorly to the propodeal pit. Comparisons of mesosomal shapes between the sexes or between males in taxa with and without this reservoir reveal that structural changes of the posterior elements of the mesosoma are associated with the presence of the large reservoirs. In turn, reservoir size is predictive of male reproductive behaviour. The probable mode of gland function, the potential use and limitations of this structure as a phylogenetic character and the relationship of gland reservoir size to the diversity of mate-locating behaviours found in the Xylocopini are discussed.