A new study published online in the journal Science suggests that both the sight and flight of the dusk-foraging hawkmoth (Manduca sexta) likely evolved to match the movements of wind-tossed flowers under low-light conditions.
A hawkmoth (Manduca sexta) clings to a robotic flower used to study the insect’s ability to track the moving flower under low light conditions. Image credit: Rob Felt / Georgia Tech.
Scientists already knew that the hummingbird-sized hawkmoths, which feed on flower nectar during the evening and at dusk and dawn, use specialized eye structures to maximize the amount of light they can capture.
But they also hypothesized that the insects might be slowing their nervous systems to make the best use of this limited light.
But if the moths were slowing their brains to see better, wouldn’t that hurt their ability to hover and track the motion of flowers?
The authors of the new study – Dr Simon Sponberg from the Georgia Institute of Technology and his colleagues from Northwest University and the University of Washington – studied this question using high-speed infrared cameras and artificial 3D-printed flowers on a robotic arm, which was programmed to move side-to-side at various frequencies.
They found that the tracking responses of Manduca sexta – referred to as the tobacco hornworm (caterpillar) or as the hawkmoth (adult) – were about 17 percent slower in dark, moonlit conditions compared to brighter, early-dusk light.
They also discovered that the movement of the artificial flower was a significant factor: when it moved at a frequency higher than 1.7 Hz, the hawkmoths had trouble tracking it. When it moved at frequencies less than 1.7 Hz, on the other hand, the moths had very little trouble.
The scientists then analyzed the movements of some of the hawkmoths’ favorite flowers as they blew in the wind, finding that 94 percent of the flowers’ motion remained below 1.7 Hz.
“We expected to see a trade-off with the moths doing significantly worse at tracking flowers in low light conditions. What we saw was that while the moths did slow down, that only made a difference if the flower was moving rapidly – faster than they actually move in nature,” Dr Sponberg said.
Taken together, these findings suggest that hawkmoths are able to avoid the pitfalls of slower visual processing because their sight is precisely adapted to the light and movement conditions of their natural environment.
“This is really an extreme behavior, though the moth makes it look simple and elegant. To maneuver like this is really quite challenging. It’s an extreme behavior from both a sensory and motor control perspective,” Dr Sponberg said.
“Seeing how well an animal with a tiny brain was able to track complicated movements and adjust its performance to different light levels was a surprising result of the work,” he said.
“This was an interesting example of how an organism can tune its brain to maintain its ability to gather food. The moths do suffer a trade-off by slowing their brains, but that trade-off doesn’t end up mattering because it only affects their ability to track movements that don’t exist in the natural way that flowers blow in the wind.”
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Simon Sponberg et al. 2015. Luminance-dependent visual processing enables moth flight in low light. Science, vol. 348, no. 6240, pp. 1245-1248; doi: 10.1126/science.aaa3042
