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By Jess Thomson
Science Reporter
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Flying insects are known to make a beeline for lights in the dark, as the saying goes, "like moths to a flame."
Now, scientists have figured out why insects are so keen on light, but it's not because they're attracted to the light source.
According to a pre-print paper on the bioRxiv preprint server titled: "Why flying insects gather at artificial light," insects don't fly directly to lights from far away because they're attracted to them, but they appear to change course toward a light if they happen to be passing by due to a strange innate biological response.
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"Our results suggest artificial lights may only trap passing insects rather than attract them directly from farther away," lead author Samuel Fabian, a postdoctoral research associate at Imperial College London, and his co-authors wrote in the paper.
The researchers described how they analyzed the flight trajectories of a number of insect species in the presence of artificial light to investigate how the insects used their dorsal-light response (DLR).
"Most flying insects display some form of the dorsal-light-response (DLR), a behavior that keeps their dorsal (top) side to the brightest visual region," the authors explained.
This DLR allows the insects to know which way is up, as the moon or sun is usually more or less directly above them, and this orientation allows them to maintain proper flight attitude and control.
The scientists found that this DLR is the reason why artificial light is so attractive to insects,—not because they're flying straight towards it, but as they're trying to keep it above them.
"Contrary to the expectation of attraction, insects do not steer directly toward the light. Instead, insects turn their dorsum toward the light, generating flight bouts perpendicular to the source," the authors wrote.
They found that at short ranges, the insects do not fly directly to a light source, but orthogonally to it, leading to orbiting and erratic flight trajectories as the light's location relative to them changes as they move. This can lead to the behavior we see insects performing: flying around a light, transfixed and somewhat trapped.
"Near artificial sources, however, this highly conserved dorsal-light-response can produce continuous steering around the light and trap an insect. Our guidance model demonstrates that this dorsal tilting is sufficient to create the seemingly erratic flight paths of insects near lights and is the most plausible model for why flying insects gather at artificial lights," the authors said in the paper.
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The fact that the insects aren't flying directly toward the light disproves one previous theory of why insects appear to be attracted to light, which was that it was some form of escape response. Another theory suggested that the insects flew towards artificial lights because they were attracted by the heat emitted by the bulb: however, the authors disproved this too, by using LED lighting, which emits negligible infrared heat radiation, but still causes insects to flock.
"Our findings suggest this light entrapment of insects at a local scale is due to a corruption of the insect's attitude control, rather than navigation," the authors concluded.
"Our experimental evidence and simulations attribute the mechanism of light entrapment to a disruption of the insect's perception of vertical rather than a navigational cue."
The authors suggested that a possible outcome of their research could be to help the construction industry to avoid the types of light that most attract insects.
"Taken together, reducing bright, unshielded, and upward facing lights will mitigate the impact on flying insects at night, when skylight cannot compete with artificial sources. Future research focused on spectral tuning of the visual components of the DLR would help isolate how best to alter artificial lights to avoid confusing insects flying at night."
The paper is still a pre-print, meaning that it has not yet been peer-reviewed and published in an established journal.
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