Dorsal landmark navigation in a Neotropical nocturnal bee.

Bees, ants, and wasps are well known to visually navigate when traveling between their nests and foraging sites. When leaving their nest, landmarks in the vicinity are memorized and used upon return to locate the nest entrance. The Neotropical nocturnal sweat bee Megalopta genalis navigates under the forest canopy at light intensities ten times dimmer than starlight. Despite these dim conditions, Megalopta is able to memorize visual landmarks around the nest entrance in the frontal visual field. Even though frontal landmarks can clearly be discerned by Megalopta, the visual feature of greatest contrast in the rainforest at night is actually the dark dorsal silhouette of the distant canopy against the brighter night sky. Several species of ants, as well as a subsocial shield bug, use bright open gaps in the canopy as dorsal landmarks to navigate home while walking. Here we show that Megalopta is also able to distinguish dorsal landmarks during homing, the first flying insect known with this capacity. Megalopta is able to discriminate between differently oriented dorsal black striped patterns, or an "artificial canopy" of black circles, and to use this information to locate its nest entrance. These results suggest that the local foliage patterns created by the canopy against the brighter sky could potentially provide the bee with reliable landmark information for navigation during foraging and homing at night. VIDEO ABSTRACT.