Morphological traits of the eyes of Lepidoptera indicate selection for binocular vision in both diurnal butterflies and nocturnal moths

This study provides a heuristic model linking gross morphological features of compound eyes in butterflies and moths and potential visual functions related to detectability, resolution and stereopsis. This framework, combined with empirical data from 2,730 specimens representing 224 species widely sampled across a tribal level phylogeny of butterflies and a recent genomic sampling of moths, supports the hypothesis that advantages of binocular vision select for morphological architecture enhancing detectability in nocturnal species and stereopsis in diurnal species. While these results are not entirely surprising given the different light environments that these species inhabit, our analysis also uncovered a strong correlation between eye size and interocular distance, particularly in diurnal Lepidoptera. In general, males have eyes that are more ventral facing than those of females, although this effect is more pronounced in some groups than others. Diurnal butterflies exhibit a striking phylogenetic pattern in the distribution of dorsal- and ventral-facing eyes that deserves additional research.