Small wasps, big muscles: Fore and hind leg modifications in chalcidoid wasps (Hymenoptera: Chalcidoidea)

A particularly conspicuous morphological feature in chalcidoid wasps are strikingly modified legs present in both males and females. It evolved convergently multiple times on either fore or hind legs implying strong evolutionary pressure and a prominent function in the wasps’ life history. We investigate the external and internal morphology of the modified legs of five species of chalcidoid wasps representing four families (Ooderidae, Heydeniidae, Chalcididae, and Leucospidae), using light microscopy, scanning electron microscopy and micro computed tomography. We aimed to identify shared characteristics as well as differences between genera/species, leg pairs and sexes and to draw first conclusions about the shared or different functions. All species and sexes share the same general leg morphology, with enlarged femur, curved tibia and a huge flexor tibiae muscle. However, there are also genus/species-specific differences such as distinctive spine-like setae on the femur of Oodera spp., or leg pair-specific differences in the position of the extensor tibiae muscle. Shared characteristics imply a common primary function in which strong forces are required to pull the tibia against the femur while differences imply different secondary functions. Both primary and secondary functions have yet to be revealed beyond informed speculations.