Variation of eye size in butterflies: inter- and intraspecific patterns

Interindividual variation of eye optics and single object resolution in bumblebees

In the eusocial bumblebees, distinct size variation occurs within the worker caste of a colony. We show that there are pronounced differences in compound eye optical quality between individual workers in Bombus terrestris. Using scanning electron microscopy and antidromic illumination techniques (the pseudopupil method), we demonstrate that large workers have extended facet diameters in conjunction with reduced interommatidial angles. Thus, both overall sensitivity and image resolution are superior in such individuals. Behavioural tests show that a 33% increase in body size is accompanied by 100% greater precision in single target detection. This improvement in spatial resolving power is much stronger than that predicted by surveying ommatidial arrays, indicating that measuring eye optics alone is insufficient for predictions of single object resolution, unless combined with behavioural tests. We demonstrate that in small bees the minimum number of ommatidia involved in target detection is seven, while in large workers a single ommatidium is sufficient for target detection. These findings have implications for foraging and division of labour in social insects.

Allometry and resolution of bee eyes (Apoidea)

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 degrees . 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.