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.