Allometry and resolution of bee eyes (Apoidea)

Abstract

A sample of compound eyes from 15 species of female pollen foraging bees (apiform Apoidea) was morphometrically analyzed. These species were chosen for size differences, different social organization, and a wide geographic and taxonomic distribution (Apidae, Megachilidae, Andrenidae, Halictidae). The results demonstrate the following characteristics for the typical compound eye in female foraging bees: (1) the vertical diameter of the eye is about twice the horizontal diameter; (2) the eyes of diurnal foragers scale isometrically with body size; (3) the eyes of three species of nocturnal foragers have about 1.8 times the surface area as compared to diurnal foragers of matching size; (4) the number of ommatidia per eye range from about 1000 in Perdita minima to about 16 000 in Xylocopa latipes; and (5) the corresponding mean interommatidial angles range from 4.7 to 1.2°. Body size, rather than species-specific ecological adaptation, is the major (97%) determinant of the number of ommatidia per eye in diurnal, as well as nocturnal foragers. The number of ommatidia per eye, and hence the visual resolution, is proportional to the square root of both body size and eye size across all species studied. The eye parameter (the product of the mean interommatidial angle and the mean lens diameter) increases slightly with decreasing body size. All this is taken as evidence that the features of the bees' visual macro-niche remained largely constant over the roughly 130 million years of their macro-evolution.