They lost track of the sky

They lost track of the sky

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The effect of reflected light depended strongly on whether it came from below or above the insect. credit: Nature Communications (2024). doi: 10.1038/s41467-024-44785-3

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The effect of reflected light depended strongly on whether it came from below or above the insect. credit: Nature Communications (2024). doi: 10.1038/s41467-024-44785-3

It’s an observation as old as humans gathering around campfires: light at night can attract an erratically circulating swarm of insects. In art, music and literature, this scene is an enduring metaphor for a dangerous but irresistible attraction. Watching their frantic movements gives the sense that something is wrong – instead of finding food and evading predators, these nocturnal fliers are trapped in the light.

Unfortunately, centuries of watching what happens has produced little certainty about why it happens. How could a simple light turn quick and precise navigators into helpless and trembling captives? We are researchers who study flight, vision, and evolution, and have used high-speed tracking techniques in research published in Nature Communications To provide an answer.

Moths to the flame?

Many ancient explanations for this hypnotic behavior have not been fully resolved. An early idea was that insects might be attracted to the heat of the flame. This was interesting, as some insects are truly thermophilic: they are attracted to fire and have evolved to take advantage of conditions in recently burned areas. But most insects found around light are not in this category, and cool lights attract them well.

Another idea is that insects are directly attracted to light, a response called phototaxis. Many insects move toward the light, perhaps as a means of escaping the darkness or their surroundings. But if this were the explanation for the clusters around the light, you might expect it to hit the source directly. This theory does little to explain the behavior of land circulation.


Credit: son

Another idea is that insects might mistake nearby light for the moon while trying to use celestial navigation. Many insects point to the moon to maintain their path at night.

This strategy is based on how objects in the distance appear to hover in place as you move along a straight path. The stationary moon indicates that you have not made any unintended turns, as you might do if you were caught in a gust of wind. However, nearby objects do not appear to follow you in the sky but drift back as you move.

Celestial navigation theory holds that insects worked to keep this light source constant, turning sharply in a failed attempt to fly straight. It’s a great idea, but this model predicts that many of the flights will turn inward toward collision, which typically doesn’t match the orbits we see. So what’s really going on?

They turn their backs on the light

To examine this question in detail, we and our colleagues captured high-speed videos of insects around different light sources to precisely 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 were not exactly identical to any existing model.


Artificial light at night interrupts the insects’ natural flight patterns. This compilation video shows a tropism behavioral model in which insects orbit around light.

Instead, a wide array of insects were constantly pointing their backs toward the lights. This is a known behavior called the dorsal light response. In nature, assuming there is more light coming down from the sky than coming from the ground, this response helps keep insects in the right direction for flight.

But turning their backs toward nearby artificial lights changes their flight paths. Just as airplanes tend to spin, sometimes rolling until the ground is almost visible outside your window, so too do flying insects spin. When their backs are turned toward a nearby light, the resulting bank circles the light, rotating but rarely colliding.

These orbital paths were just one of the behaviors we observed. When insects fly directly under a light, they often arc upward as they pass behind it, keeping their backs to the lamp until they finally fly straight up, then stop and fall out of the air. Even more alarming, when flying directly above the light, the insects tend to turn upside down, turning their backs back to the light but then suddenly crashing.

more information:
Samuel T. Fabian et al., Why do flying insects congregate in artificial light? Nature Communications (2024). doi: 10.1038/s41467-024-44785-3

Magazine information:
Nature Communications

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