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.