Unique morphological and morphometric traits of nocturnal bee antennae

Crepuscular and/or nocturnal bees are rarely sampled and consequently poorly studied. These bees have exceptional adaptations concerning their visual system to navigate under low light conditions. However, their foraging success may as well be associated with an enhanced chemoreception. In fact, nocturnal bees find flowers in the dark based on the strong scent released by plants at night, which suggests special adaptations of the bees’ sense of smell. Our study aimed at comparing olfaction-related morphological and morphometric traits between nocturnal (six species) and diurnal bees (five species) in Brazil. In addition to determining the length of the species’ flagella and flagellomeres, we used scanning electron microscopy to classify the antennal sensilla and to estimate their density. Despite a general decrease in relative flagellum length with increasing body size (intertegular distance), nocturnal bees had longer flagella and flagellomeres than diurnal bees. Moreover, sensilla associated with CO₂ detection, mechanoreception, and chemoreception were larger in size, number, and density in nocturnal than in diurnal species. Our findings suggest that, during evolutionary history, the nocturnal environment has selected bees with morphological features that improve scent perception, which is crucial for the foraging activities under dim light conditions.