The sweat bee Megalopta genalis, a Neotropical nocturnal bee species that navigates under the forest canopy at light intensities 10 times dimmer than starlight, is able to learn dorsal landmarks to find its nest during homing, the first flying insect known with this capacity.
A female sweat bee Megalopta genalis. Scale bar – 2 mm. Image credit: Ajay Narendra.
Megalopta genalis is a species in the family Halictidae (commonly referred to as sweat bees), the second-largest family of bees. The term ‘sweat bee’ refers to their attraction to human sweat and perspiration.
First described in 1916, Megalopta genalis is native to Central and South America.
The special compound eyes of this species are 30 times more sensitive to light and concentrate photons of light in a way day-flying honeybee eyes cannot.
“Megalopta genalis is a facultatively eusocial tropical bee that nests in hollowed-out wooden sticks entangled in the understory of the forest,” said University of Lund’s Dr. Eric Warrant and colleagues.
“Typically, each bee forages during roughly 70 min windows bordered by astronomical twilight, once during dusk and once during dawn.”
“Due to the dense foliage in the rainforest, the light intensity during these twilight periods is typically around that of a starlit night under an open sky.”
The researchers carried out several experiments at the Smithsonian Tropical Research Institute’s research station on Barro Colorado Island in Panama.
They first set up a special bee motel. Imagine the entrances to five rooms: each entrance is an identical, circular gray disc. The door is a tiny round opening at the center of the disc — the end of a stick nest.
In the first experiment, they asked if the bees could find their way home using a single landmark.
The scientists placed a black bar above the entrance to one of the nests. After the bees left to look for pollen, they changed the order of the nests, but the real nest was still marked by the black bar above the entrance. When the bees returned, they chose the right nest.
Next, the authors made it a bit more complicated by building a sort of awning — like the roof of a carport — above each nest entrance.
Over the four empty nests they placed an awning with a pattern of black and white bars, and over the occupied nest, a different pattern.
Again, when the bees went out, the team changed the position of the nest, and again, they found the right nest when they came back.
But the researchers still had not ruled out the possibility that the bees were using another clue, like smell, to find their own nest.
So they left the nest in the same place, but put the bee’s learned pattern over an empty nest and the bees chose the empty nest, showing that they were using the pattern as a guide, not smell or some other signal from the nest.
The team’s final experiment was the most elegant: this time two nests shared a common awning, a design of dark circles on a light background made to represent the patterns of light and dark in real forest canopies.
The entrance to an empty nest was located under one end of the awning, and the entrance to the occupied nest under the opposite end. This time, when the bees left, the team flipped the awning around so that the pattern was reversed.
The bees chose the wrong nest more than 80% of the time, showing once and for all that they were navigating based on the pattern above.
“Our 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,” the scientists said.
The findings were published in the journal Current Biology.
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Sandra Chaib et al. Dorsal landmark navigation in a Neotropical nocturnal bee. Current Biology, published online June 10, 2021; doi: 10.1016/j.cub.2021.05.029
