Compound-eye optical diversity across South American butterflies

According to their ecological niche, butterfly compound eyes show regional variation in optical sampling and spectral sensitivity. We examined how optical architecture varies both across homologous eye regions and among six South American butterfly species selected to represent contrasting activity periods (diurnal and crepuscular) and feeding habits (flower- and fruit-feeding). By studying eyeshine reflections at multiple positions along the vertical and anteroposterior axes of the eye, we characterised pseudopupil shape and quantified its area, facet size and interommatidial angles. The pseudopupil was large and vertically elongated in the frontal region, indicating high resolving power and differences between vertical and horizontal sampling resolution. It became smaller and rounder towards the back and in dorsal and ventral regions. Facets were also largest and interommatidial angles were smallest frontally, consistent with greater light-gathering capacity and finer spatial sampling, respectively. Notably, Opsiphanes invirae, a crepuscular fruit-feeding butterfly, showed the largest facet diameter among the species examined, providing a larger optical aperture and thereby increasing the amount of light captured by each ommatidium under dim-light conditions. Current results provide the first examples of how ecological factors shape the visual systems of South American butterflies, whose eye diversity remains largely unexplored.