It’s an observation as old as people gathering around campfires: At night, light can attract an erratically circling crowd of insects. In art, music and literature, this spectacle is an enduring metaphor for dangerous but irresistible attractions. And watching their frantic movements really gives the feeling that something is wrong: that instead of finding food and avoiding predators, these nocturnal pilots are trapped in a light.
Unfortunately, centuries of witnessing what is happening have provided little certainty as to why it is happening. How does a simple light turn fast, precise navigators into helpless, fluttering prisoners? We’re researchers studying flight, vision and evolution, and we’ve used high-speed tracking techniques in newly published research to provide an answer.
Moths to a flame?
Many ancient explanations for this hypnotic behavior have not yet been fully elucidated. An early idea was that the insects might be attracted to the heat of a flame. This was interesting, because some insects are truly pyrophilic: they are attracted to fire and have evolved to take advantage of the conditions in recently burned areas. But most insects around a lamp don’t fall into this category, and cool lamps attract them quite well.
Another thought was that insects were simply attracted directly to light, a response called phototaxis. Many insects move toward light, perhaps as a way to escape the dark or oppressive environment. But if this were the explanation for the clusters around a lamp, you would expect them to collide directly with the source. This theory does little to explain the wild circling behavior.
Yet another idea was that insects might mistake a nearby light for the moon while trying to use celestial navigation. Many insects refer to the moon to keep their course at night.
This strategy is based on the way distant objects appear to float in place as you follow a straight path. A stable moon indicates that you have not made any unintended turns, as you might do if you are buffeted by a gust of wind. However, objects that are closer do not appear to follow you in the air, but drift behind you as you move past.
Celestial navigation theory posited that insects worked to keep this light source steady, turning sharply in an unsuccessful attempt to fly straight. An elegant idea, but this model predicts that many flights will turn inwards and lead to a collision, which usually does not match the trajectories we see. So what’s really going on?
They turn their backs to the light
To investigate this question in detail, we and our colleagues captured high-speed videos of insects around different light sources to accurately determine flight paths and body postures, both 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 match well with any existing model.
Instead, a wide swath of insects consistently pointed their backs to the lights. This is a well-known behavior called the dorsal light response. In nature, assuming more light comes from the sky than from the ground, this response helps keep insects in the correct orientation for flight.
But turning their backs to nearby artificial light changes their flight paths. Just as airplanes turn to turn, sometimes rolling until the ground seems almost straight out your window, flying insects also turn. When their backs are to a nearby light, the resulting bank leads them around the light, circling but rarely colliding.
These track-like paths were just one of the behaviors we observed. When insects flew directly under a lamp, they often arched upward as it flew behind them, keeping their backs to the lamp until they finally flew straight up, came to a stop and fell out of the sky. Even more impressive, when insects flew directly above a light, they tended to turn upside down, once again turning their backs on the light, but then crashing abruptly.
Why a dorsal light response?
Although light at night can harm other animals – for example by distracting migratory birds into urban areas – larger animals do not appear to lose their vertical orientation. So why do insects, the oldest and most species-rich group of kites, rely on a response that makes them so vulnerable?
It may have something to do with their small size. Larger animals can sense gravity directly with sensory organs attracted to the acceleration, or any acceleration. For example, humans use the vestibular system of our inner ear, which regulates our sense of balance and usually gives us a good idea of which way is down.
But insects have only small sensory structures. And especially when performing fast flight maneuvers, acceleration provides only a poor indication of the direction down. Instead, they seem to be betting on the clarity of the sky.
Before modern lighting, the sky was usually brighter than the ground, day and night, and thus a reasonably reliable signal for a small active flyer hoping to maintain a stable orientation. Artificial light that sabotages this ability by causing insects to fly in circles is relatively recent.
The growing problem of nighttime lighting
As new technology spreads, the lights that permeate the night are spreading faster than ever. With the introduction of cheap, bright, broad-spectrum LEDs, many areas, such as major cities, will never see a dark night again.
Insects aren’t the only creatures affected. Light pollution disrupts circadian rhythms and physiological processes in other animals, plants and humans, often with serious health consequences
But insects trapped around a light seem to get the worst of it. Unable to obtain food, easily spotted by predators and prone to exhaustion, many die before morning breaks.
In principle, light pollution is one of the easiest things to solve, often by simply flipping a switch. Limiting outdoor lighting to useful, focused warm light, no brighter than necessary and no longer than necessary, can significantly improve the health of nocturnal ecosystems. And the same practices that are good for insects are helping restore visibility to the night sky: More than a third of the world’s population lives in areas where the Milky Way is never visible.
While insects circling a lamp are a fascinating sight, it is certainly better for the insects and the benefits they provide to humans if we leave the night unlit and let them perform the activities they perform so masterfully under the night sky.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Samuel Fabian, Imperial College London; Jamie Theobald, Florida International Universityand Yash Sondhi, University of Florida.
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.