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“Flying insects don’t fly directly to lights from far away because they’re attracted to them, but appear to change course toward a light if they happen to be passing by due to a strange inborn biological response,” writes Samuel Fabian, a bioengineer, in a research paper.

Until now, the leading scientific hypothesis has been that insects use the moon’s light to direct the way at night and mistake artificial lights for the moon. But this idea doesn’t explain why insects that only fly during the day also gather around lights.

To find out what really happens, Samuel’s team track the precise movements of insects in the wild around lights using a high-speed camera. This revealed two notable behaviours. First, when insects fly above lights, they often invert (转向) themselves and try to fly upside down, causing them to fall very fast. Just after insects pass under a light, they start doing a ring road. As their climb angle becomes too steep, they suddenly stop and start to fall. Second, when insects approach a light from the side, they may circle or “orbit” the light.

The videos show that the inversions sometimes result in insects falling on lights. It can appear to the naked eye as though they are flying at the lights. “Instead, insects turn their dorsum toward the light, generating flight perpendicular(垂直) to the source,” the team write. It is common to the two behaviours that the insects are keeping their backs to the light, known as the dorsal light response (DLR). This DLR is a shortcut for insects to work out which way is up and keep their bodies upright, as the moon or sun is usually more or less directly above them, and this direction allows them to maintain proper flight attitude and control. They also find that the insects fly at right angles to a light source, leading to orbiting and unstable flights as the light’s location relative to them changes as they move.

Samuel’s team suggest that a possible outcome of the research could help the construction industry to avoid the types of light that most attract insects.