Conserved optical adaptations enable amphibious vision in compound eyes.

The visual systems of amphibious animals must perform in two optically distinct environments: air and water. Little is known about how diffraction-limited, arthropod compound eyes maintain optical function across the air-water interface, aside from one commonality: flat corneas. Since the external shape of a lens is only one of several properties that influence optical performance, we examined how the corneal optics of compound eyes varied with amphibious ecology across two biological transects: developmentally in one insect species and comparatively among multiple species. Both analyses revealed that flat corneas alone are insufficient to provide amphibious optical performance. The corneas of compound eyes universally combine flat external surfaces, rounded internal surfaces and inwardly expanded thicknesses to support visually guided behaviours in air and water. Conservation of these features among diverse groups provides a strong evolutionary link between the optical performance of compound eyes and the environmental medium.