Evolution of Peripheral Visual System in the Apoidea: A Role for Food Item Mobility?

The morphology of insect visual system was often linked with resource type and resource-finding strategies. Since larger compound eyes and ocelli altogether improve image resolution, capturing light and motion perception, one may expect that insects specialised in chasing very mobile resources possess such morphological optimisation. We tested this hypothesis in females of Apoidea (Hymenoptera), which include bees (which just land on flowers to collect pollen), wasps hunting for weakly mobile prey that are not chased in flight (e.g., beetles, aphids) (LMr-species) and wasps which prey upon highly-mobile flying prey (e.g., flies, bees) (HMr-species). By analysing 77 species, we have found that, once taken into account the head width, HMr- and LMr-wasps did not differ in eye size, but had larger eyes than bees. Median ocellus diameter, on the other hand, did not differ between bees and wasps, though HMr-wasps had larger ocelli than LMr-wasps. The observed variations were largely dependent on head width, with the eye enlargement being faster in wasps than in bees as the head size increased. Phylogenetically-corrected models highlighted a strong effect of common ancestry on morphological variation but confirmed the significant effect of food mobility for the relative eye size. Ancestral state reconstructions suggested a shift to relatively smaller eyes in correspondence with the shift to pollen provisioning, while things seemed to be more complex with the relative ocellar size, likely because of the negative allometry of this trait, especially strong in bees. We may conclude that the need to hunt rapidly moving prey in flight has likely contributed to the evolution of the visual system in Apoidea.