Solar–absorber antireflector on the eye of an Eocene fly (45 Ma)

Antireflection structures on eyes potentially increase visual efficiency through increased photon capture for a given stimulus condition. We report an unusual surface grating on the compound eyes of an Eocene dolichopodid dipteran (45 Ma) known only from Baltic amber. By measuring the reflective properties of a flat model of this grating constructed from material of appropriate refractive index we show that this 'fly eye grating' is an efficient antireflector of white light at angles up to 60 degrees, and a relatively good antireflector for angles beyond 60 degrees. We calculate that such a grating would be particularly useful on a curved corneal surface as it would increase the transmission of incident light through the cornea compared with a smooth surface. This structure is also reported on the curved regions of the corneas of at least two extant dipterans. We argue that this grating probably derives from the previously described moth eye antireflection structure, which we also report here to occur in a silverfish, a 'primitive' insect. The fly eye grating is a more efficient antireflector than the moth eye structure only at angles greater than about 10 degrees from the surface normal. A comparable antireflector is employed on solar absorbers.