Differences in caste dimorphism among three hornet species (Hymenoptera: Vespidae): forewing size, shape and allometry.

Caste shape dimorphism (CShD) has previously been studied in wasps through comparison of different body parts, originating from different imaginal discs. Using geometric morphometrics with a new protocol for measuring wings of pinned specimens from natural history collections, we tested CShD of three hornet species in an organ developed from a single imaginal disc: the forewing. Gaussian mixture models retrieved most castes and species levels, confirming that caste is an important component of wing variations in females of these hornets. Size and allometry - the influence of size on shape - contribution to wing dimorphism between castes was major, but failed to explain the entire shape dimorphism. This deviation from simple allometric scaling was not similar in the three species: in Vespa tropica, allometric directions in the shape space differed between castes, whereas in V. crabro and V. velutina, they were similar but a significant part of CShD resulted from lateral transpositions. These results clearly indicate that queens are not just enlarged workers. They also support that the different patterns of CShD may result from different developmental mechanisms. Finally, they highlight that even in a highly social group like hornets, there is still variation in caste dimorphism among species.