It’s an observation as old as humans gathering around campfires: Light at night can attract a multitude of insects circling erratically.
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 evading predators, these night pilots are stuck to a light.
Unfortunately, centuries of observing what happens have produced little certainty about why this happens.
How does a simple light source transform fast, precise navigators into helpless, fluttering captives? We are researchers examining animal flight, vision, and evolution, and we used high-speed tracking techniques in newly published research to provide an answer.
Moths around a flame?
Many ancient explanations for this hypnotic behavior have not been fully proven.
An early idea was that insects might be attracted to the heat of a flame. This was interesting because some insects actually are pyrophilic: they are attracted to fire and have evolved to take advantage of conditions in recently burned areas. But most insects around a light are not in that category, and cool lights attract them very well.
Another assumption was that insects were only directly attracted to light, a response called phototaxis. Many insects move toward the light, perhaps as a way to escape dark or imprisoning environments. But if that were the convincing explanation, one would expect them to crash directly into the source. This theory does not explain wild circling behavior.
Still another idea was that insects could mistake a nearby light for the Moon when trying to use celestial navigation. Many insects use the Moon as a reference to maintain their path at night.
This strategy is based on the way objects in the distance appear to hover in place as you move along a straight path.
A stable Moon indicates that you have not made any unintentional turns, as would happen if you were caught in a gust of wind. Closer objects, however, do not appear to follow you across the sky, but fall behind as you pass.
Celestial navigation theory held that insects struggled to keep this light source stable, turning sharply in a failed attempt to fly in a straight line.
It’s an elegant idea, but this model predicts that many flights will spiral toward a collision, which generally doesn’t match the orbits we see. So what’s really going on?
Turning your back to the light
To examine this question in detail, we and our colleagues captured high-speed videos of insects around different light sources to accurately determine their flight paths and body postures.
This study was carried out both in the laboratory at Imperial College London and at two field sites in Costa Rica, CIEE and Estación Biológica. We found that the insects’ flight patterns were not compatible with any existing models.
Instead, a wide swath of insects consistently pointed their backs toward the lights. This is a behavior known as the dorsal light response. In nature, assuming more light falls from the sky than rises from the ground, this response helps keep insects in the proper orientation for flight.
But pointing their backs at nearby artificial lights changes their flight paths.
Just as planes tilt to make turns, making the ground appear almost straight through the window, tilted insects also make curves.
When their backs orient toward a nearby light, the resulting tilt causes them to rotate around the light, circling but rarely colliding.
These orbiting 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 to the lamp until, finally, flying directly upward, they stopped and fell into the air.
And, even more interesting, when flying directly into a light, the insects tended to turn upside down, again turning their backs to the light but falling abruptly.
Why a dorsal light response?
Although night light can harm other animals – for example, by diverting migratory birds to urban areas – larger animals do not appear to lose their vertical orientation.
So why do insects, the oldest and most species-rich group of fliers, rely on a response that leaves them so vulnerable?
Maybe it has to do with its small size. Larger animals can sense gravity directly with sensory organs pulled by their acceleration, or by any acceleration.
We humans, for example, use the vestibular system of our inner ear, which regulates our sense of balance and generally gives us a good sense of which direction is down.
But insects have only small sensory structures. And, especially when performing rapid flight maneuvers, acceleration provides only a poor indication of the downward direction. Instead, they seem to bet on the brightness of the sky.
Before modern lighting, the sky was generally brighter than the ground, day or night, and so provided a fairly reliable track for a small active flyer hoping to maintain a stable orientation.
Artificial lights that sabotage this ability, causing insects to fly in circles, are relatively recent.
The Growing Problem of Night Lighting
As new technologies spread, the lights that permeate the night are proliferating faster than ever. With the introduction of cheap, bright, broad-spectrum LEDs, many areas, such as large cities, have never seen a dark night.
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 suffer most from this. Unable to obtain food, easily detected by predators and prone to exhaustion, many die before dawn.
In principle, light pollution is one of the easiest things to fix, often just flipping a switch. Restricting outdoor lighting to useful, targeted warm light, no brighter than necessary and for no longer than necessary, 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 the Milky Way is never visible.
Although insects circling around a light are a fascinating spectacle, it is certainly better for them and the benefits they provide to humans when we leave the night unlit and allow them to carry out the activities they so masterfully perform under the sky. nocturnal.
* Samuel Fabiano is a postdoctoral research associate in Bioengineering at Imperial College London. Jamie Theobald is an associate professor of Biological Sciences at Florida International University. Yash Sondhi is a postdoctoral research associate in Entomology at the Mcguire Center for Lepidoptera & Biodiversity at the Florida Museum of Natural History at the University of Florida.
This article was published in The Conversation and reproduced by BBC News Brasil under a Creative Commons license. Click here to read the original version