The Incredible Reason The Incredible Reason Why Insects Fly Around Light

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It's an observation as old as human gatherings around bonfires: nighttime light can attract hordes of erratically fluttering insects. In art, music and literature, this spectacle is an enduring metaphor for dangerous but irresistible attractions. Watching their frenetic movements really gives the feeling that something is wrong: that instead of searching for food and evading predators, these nocturnal pilots are trapped by a light.

Although we have been witnessing their behavior for centuries, we have little certainty about why they do it. How is it possible that a simple light turns fast and precise navigators into helpless captives and clumsy flutters?

Our research team examines the flight, the vision and the evolution of insects, and we have used high-speed tracking techniques in research recently published in Nature.

Moths towards the flame

Many of the old explanations for this hypnotic behavior have not yet come together. One of the first ideas was that insects were attracted to the heat of the flame. This was interesting, since some insects are actually pyrophiliacs: They are attracted to fire and have evolved to take advantage of the conditions of recently burned areas. But most insects that hover around a light don't fall into this category, and cold lights attract them too.

Another idea was that insects were attracted directly to light, a behavior called phototaxis. Many insects move toward light, perhaps as a way to escape dark environments or traps. But if this were the explanation for clusters around a light, one would expect them to collide directly with the source. But it doesn't happen like that. They don't go straight to the light, they fly in circles.

The most romantic theory is that insects could mistake a nearby light for the Moon, when trying to use the celestial navigation. Many insects use the Moon to stay on track at night.

This strategy is based on the way objects located at a great distance appear stationary while you are moving in a straight line. A fixed Moon indicates that you have not made any involuntary turns, as might happen if you were buffeted by a gust of wind. However, closer objects do not appear to follow us in the sky, but rather fall behind as one moves forward.

The theory of celestial navigation held that insects strove to keep this light source stable, turning sharply in an unsuccessful attempt to fly straight. It's an elegant idea, but this model predicts that many flights should spiral toward a collision, which doesn't usually match the orbits we see. So what's really going on?

Multiple cameras in a forest environment at night.
The scientists used high-speed stereo motion capture to document how the presence of artificial light at night affects the flight behavior of insects. Samuel Fabian, CC BY-ND

We record thousands of flights

To examine this question in detail, we captured high-speed videos of insects around different light sources. We seek to accurately determine flight trajectories and body postures, both in the laboratory of the Imperial College London, as in two field sites in Costa Rica, the CIEE and the Biological Station. And we discovered that their flight patterns did not fit any existing model.

Rather, a large number of insects systematically placed their backs towards the lights. This is a behavior known as dorsal response to light. In nature, assuming that light comes down more from the sky than up from the ground, this response helps keep insects in the proper orientation for flight.

Placing their backs toward nearby artificial lights alters their flight paths. They do something like airplanes when they tilt to turn, sometimes until the ground looks almost straight out the window. When their backs are oriented toward a nearby light, the resulting motion causes them to spin around the light, circling but rarely colliding.

These orbital trajectories were just one of the behaviors we observed. When insects flew directly under a light, they often arched upward as it passed behind them, keeping their backs facing the light until, finally, flying in a straight line, they stopped and fell into the void.

And we found something even more extraordinary: when flying directly above a light, the insects tended to turn upside down, facing away again, and then crash sharply.

The diagrams show the insects rotating vertically or horizontally or inverting in the presence of artificial light.
Three different turning behaviors observed in which flying insects turn their backs to artificial light. Jamie Theobald, CC BY-ND

Why turn your back on the light?

Although night light can harm Other animals –for example, at divert migratory birds to urban areas– larger animals do not seem to lose their vertical orientation.

So why do insects, the oldest and most species-rich group of flyers, resort to a response that leaves them so vulnerable?

Maybe it has to do with its small size. Larger animals can feel gravity because they have sensory organs that perceive their acceleration, or any acceleration. Humans, for example, use vestibular system of our inner ear, which regulates our sense of balance and usually gives us a good idea of which way the ground is.

But insects only have small sensory structures. And, especially when performing rapid flight maneuvers, acceleration only gives them a poor indication of where they are going. Instead, they seem to rely on the luminosity of the sky.

Before modern lighting, the sky was usually brighter than the ground, day or night, so it provided a fairly reliable signal for a small active flyer hoping to maintain a constant orientation. Artificial lights that sabotage this ability, prompting insects to fly in circles, are relatively recent.

The growing problem of night lighting

As new technologies spread, the lights that permeate the night proliferate faster than ever. With the introduction of cheap, bright, and spread spectrum, many areas, such as large cities, never see a dark night.

A view up through the treetops towards a dark, starry sky, with a bright light at the top of the screen coming from a light bulb near the ground.
This image at the authors' field research site in Monteverde, Costa Rica, shows how artificial light competes with the night sky. Samuel Fabian, CC BY-ND

Insects are not the only ones affected. Light pollution alters circadian rhythms and other physiological processes animals, plants and humans, often with serious health consequences.

But insects trapped around a light seem to get the worst of it. Unable to find food, easily found by predators, and prone to exhaustion, many die before morning arrives.

In principle, light pollution is one of the easiest things to solve, it is often enough to turn off a switch. Limit outside lighting to a warm, useful and specific light, no brighter than necessary and for less time, can greatly improve the health of nocturnal ecosystems. And the same practices that are good for insects help restore vision of the night sky: more than a third of the world's population lives in areas where Milky Way is never visible.

Although insects circling a light are a fascinating sight, it is certainly better for them and the benefits they bring to humans that the night is dark and that we let them freely carry out all those activities that they carry out so masterfully under a sky in which the only thing that shines are the stars.


Article translated thanks to the collaboration with Lilly Foundation.The Conversation


Samuel Fabian, Postdoctoral Research Associate in Bioengineering, Imperial College London; Jamie Theobald, Associate Professor of Biological Sciences, Florida International University and Yash Sondhi, Postdoctoral Research Associate in Entomology, Mcguire Center for Lepidoptera & Biodiversity, Florida Museum of Natural History, University of Florida

This article was originally published in The Conversation. read the original.

Octavio Alonso