Light intensity limits foraging activity in nocturnal and crepuscular bees

A crepuscular or nocturnal lifestyle has evolved in bees severaltimes independently, probably to explore rewarding pollen sourceswithout competition and to minimize predation and nest parasites.Despite these obvious advantages, only few bee species are nocturnal.Here we show that the sensitivity of the bee apposition eyeis a major factor limiting the ability to forage in dim light.We present data on eye size, foraging times, and light levelsfor Megalopta genalis (Augochlorini, Halictidae) in Panama,and Lasioglossum (Sphecodogastra) sp. (Halictini, Halictidae)in Utah, USA. M. genalis females forage exclusively during twilight,but as a result of dim light levels in the rain forest, theyare adapted to extremely low intensities. The likely factorlimiting their foraging activity is finding their nest entranceon return from a foraging trip. The lowest light intensity atwhich they can do this, both in the morning and the evening,is 0.0001 cd m^–2. Therefore, they leave the nest at dimmerlight levels in the morning than in the evening. Lasioglossum(Sphecodogastra) foraging is limited by light intensity in theevening, but probably by temperature in the morning in the temperateclimate of Utah. We propose that the evolution of nocturnalityin bees was favored by the large variance in the size of females.