by Turning their backs on the light
To examine this question in detail, we and our colleagues took high-speed videos of insects around different light sources to accurately determine flight paths and body posture, in the laboratory at Imperial College London and at two field sites in Costa Rica, CIEE and the Estación Biológica. We found that their flight patterns did not closely match any existing model.
Instead, a wide variety of insects consistently pointed their backs toward the lights. This is a well-known behavior known as the backlight response. In nature, assuming that more light comes down from the sky than up from the ground, this response helps keep insects in the proper orientation for flight.
But pointing their backs towards nearby artificial lights changes their flight paths. Just as airplanes turn, sometimes rolling until the ground can be seen almost straight out your window, banking insects also turn. When their backs are aimed at a nearby light, the resulting stream bends them around the light, circling but rarely colliding.
These orbital paths were just one of the behaviors we observed. When insects were flying directly under a light, they often went up as it passed behind them, holding their backs against the bulb until they stopped and fell out of the air flying straight up. And even more strongly, when flying directly above a light, insects tended to go upside down, turning their backs again to the light but then suddenly crashing.
Why is there a backlight answer?
Although light at night can harm other animals – for example, by diverting migratory birds to urban areas – large animals do not seem to lose their vertical orientation. So why do insects, the oldest and most species-rich group of flyers, rely on a response that leaves them so vulnerable?
It may have something to do with their small size. Animals can sense gravity more directly with sensory organs that detect their acceleration, or any acceleration. Humans, for example, use the vestibular system of our inner ear, which controls our sense of balance and usually gives us a good sense of the way down.
But insects only have small sensory structures. And especially when performing fast flight maneuvers, acceleration only gives a bad signal for the route below. Instead, they seem to have bet on the brightness of the sky.
Before modern lighting, the sky was usually brighter than the ground, day or night, so it provided a fairly reliable cue for a small active flyer hoping to maintain a fixed orientation. Artificial lights that sabotage this ability, by cueing insects to fly in circles, are relatively recent.
The growing problem of night lighting
As new technology spreads, lights that go through the night are proliferating faster than ever. With the introduction of cheap, bright, broad-spectrum lights, many areas, such as large cities, do not see a dark night.
Insects are not the only creatures affected. Light pollution disrupts circadian rhythms and physiological processes in animals, plants and other humans, and often has serious health consequences.
But insects caught around the light seem to get the worst of it. Unable to secure food, easily seen by predators and exhausted, many die before morning.
In principle, light pollution is one of the easiest things to fix, often by flipping a switch. The health of nocturnal ecosystems can be greatly improved by restricting outdoor lighting to useful, targeted warm light, no brighter than necessary, and for no longer than necessary. And the same practices that are good for insects help restore views of the night sky: More than a third of the world’s population lives in areas where the Milky Way is never visible.
Although insects that go around the light are a great spectacle, it is definitely better for the insects and the benefits they bring to humans when we leave the night without light and let them go about their activities masterfully under the night sky.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Samuel Fabian, Imperial College London; Jamie Theobald, Florida International Universityand Yash Sondhi, University of Florida.
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Samuel Fabian receives funding from the European Research Council and a National Geographic Explorer Grant.
Jamie Theobald receives funding from the National Science Foundation and the US Air Force Office of Scientific Research.
Yash Sondhi receives funding from the Florida International University Graduate School, the Susan Levine Foundation, a National Geographic Explorer Grant, the American Philosophical Society, and the Kimberly-Green Latin American and Caribbean Center.
