New research used motion capture to reveal insects don’t fly directly toward lights but tilt their backs toward the source, trapping them in loops.This “dorsal light response” helps insects orient themselves in space, but near lights it backfires.The findings explain the familiar sight of insects circling lights, and could inform efforts to reduce light pollution’s impacts.Researchers suggest that reducing upward-facing lights and ground reflections could help night-flying insects.
Flying insects’ mysterious attraction to flames and artificial lights has intrigued scientists and the public for centuries. The age-old phrase “drawn like a moth to a flame” captures moths’ perplexing tendency to circle lights at night. But it’s not just moths — flies, wasps and other insects also spiral around lamps and crash into lit surfaces after dark.
Now, new research reveals the mechanism behind this phenomenon. Scientists tracked insects’ movements in 3D around lights using motion-capture cameras and stereo videography. They found that rather than steering straight toward lights, insects tilt their backs toward the light source. This reflex gone awry traps them in looping, circular flights.
“What we kept finding is that insects, such as dragonflies, moths, butterflies and other night-flying insects as well, were tilting their backs, which we call the dorsal axis, their dorsum, towards the light,” lead researcher Samuel Fabian, an entomologist at Imperial College London, said in an interview for Nature.
This “dorsal light response” helps insects orient themselves in 3D space. “It’s quite strange for us to think about this, because as animals that spend most of our time on the ground, it’s quite obvious which way gravity is. But actually, if you’re flying you’re pulling all kinds of g-forces, and those g-forces, those accelerations as you’re moving around can kind of mask exactly where true gravity is,” Fabian said.
Before artificial lights, gazing skyward was a handy gauge of which way was up. As such, insects tilt their backs toward the brightest visual area. Near lights, however, this innate reflex backfires. “That means that all of their flight forces they’re producing, the lift, is they’re not pointing in the right direction for them to continue flight, they’re going to start curving. And so that’s why we see them circling, and not really spiraling in, cruising around and round and round and seeming completely stuck and unable to leave,” Fabian said.
Other proposed explanations, like heat-seeking behavior or navigating by moonlight, fail to match the 3D flight patterns revealed in this research. However, Fabian said additional factors may draw insects to lights from farther away. His team only examined short-range interactions within a few meters of light sources.
The findings shine a light on behavior familiar to anyone who has lingered near a lamppost or patio light at night. More importantly, unraveling this mechanism could inform conservation, especially efforts to reduce light pollution’s heavy toll on insects. For example, researchers suggest that reducing upward-facing lights and ground reflections could help night-flying insects.
Many studies over the past decade all point to a serious, dramatic decline in insect abundance across the globe. Plummeting insect populations could deeply impact ecosystems and human civilization, as these tiny creatures form the base of the food chain, pollinate the crops we eat, dispose of waste, and enliven soils.
“How could we change lighting environments to not trap insects because we’re facing a massive decline in insects around the world, and artificial light at night is one of the factors that could potentially be leading to this decline,” Fabian said. Altering lighting to avoid wavelengths that trigger the dorsal light response could reduce impacts on fragile insect populations.
“It’s remarkable how an innate and adaptive behaviour, through which an insect positions itself so its back faces the light and hence keeps a steady flight path, becomes maladaptive close to strong point sources such as lamps,” Gareth Jones of the University of Bristol, who wasn’t involved in the research, told The Guardian. “The findings suggest the large numbers of insects that congregate at street lights are trapped there by orbiting the lamps.”
So next time you watch a mesmerized moth endlessly looping a porch light, know it’s not chasing the bulb, but is just lost in the spin of a reflex gone wrong. This timeless mystery now has an explanation grounded in insect biology and behavior.
Banner image of the moths of the Intag Valley lit on a white sheet at night. Photo courtesy of Carlos Zorrila.
Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in ecology and evolutionary biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.
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Citation:
Fabian, S. T., Sondhi, Y., Allen, P. E., Theobald, J. C., & Lin, H. (2024). Why flying insects gather at artificial light. Nature Communications, 15(1), 689. doi:10.1038/s41467-024-44785-3
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