On the possible function of the proximal lens unit of Notonecta glauca

The water bug Notonecta glauca has doublet lenses corrected for spherical aberration. The proximal lens unit constitutes a refractive index transition between the distal lens unit and the crystalline cone. It inevitably reduces the reflectivity of the eye. Such correction for spherical aberration, however, could also be simply achieved by means of an aplanatic singlet lens. Therefore it seems possible to suppose that the proximal lens unit might produce both decreased reflection and increased transmission. To test this idea, the reflectivity of the doublet in Notonecta is calculated and compared with the reflectivity of two corresponding aplanatic singlets of different refractive indices for contact with air and water. Because of the lower lens unit, the absolute reflection suppression delta R in the dioptric apparatus is about 2% in water and the relative reflectivity reduction delta R/R amounts to 65%. In air this effect is negligible. The question of the evolutionary importance of this phenomenon is briefly discussed.