ScienceDaily (Aug. 16, 2004) — Day-active bees, such as the honeybee, are well known for using visual landmarks to locate a favoured patch of flowers, and to find their way home again to their hive. Researchers have now found that nocturnal bees can do the same thing, despite experiencing light intensities that are more than 100 million times dimmer than daylight. The new findings, reported in the latest issue of Current Biology by a team led by Eric Warrant at Lund University, Sweden, advance our understanding of the visual powers of nocturnal animals.

The competitive and dangerous world of the tropical rainforest has driven many normally day-active animals to adopt a nocturnal lifestyle, with the cover of darkness allowing them to exploit food resources in relative peace. Several groups of bees and wasps – including the Central American halictid bee Megalopta genalis – have become nocturnal, and despite the darkness and their apparently insensitive compound eyes, they have retained remarkable visual abilities. In the new work, performed on Barro Colorado Island in Panama, the researchers used infrared night-imaging cameras to show that by performing special orientation flights, Megalopta visually learns landmarks around the nest entrance prior to foraging and uses these landmarks to locate the nest upon return. The researchers found that if landmarks were moved to a nearby site while the bee was away, upon her return she intently searched for her nest in the landmark-bearing, but wrong, location.

Despite this impressive behavioral sensitivity, optical and physiological measurements revealed that Megalopta’s eyes are only about 30 times more sensitive to light than those of day-active honeybees, woefully inadequate to account for Megalopta’s nocturnal homing abilities. A solution to this paradox may lie outside the eye. The researchers identified in the bee’s brain specialised visual cells with morphologies suited to summing light signals and intensifying the received image.

The head of the nocturnal sweat bee Megalopta genalis (Family Halictidae) features large compound eyes, sensitive antennae that bristle with receptors detecting vibrations and odours, powerful mandibles for burrowing in wood and three round ocelli, small eyes whose function in nocturnal bees is unknown. Scanning electron microscope image by Rita Wallén. (Courtesy of Lund University, Sweden)

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