For millennia, people have watched nocturnal insects flying chaotically around campfires and lamps. Until now, it has been claimed that they use moonlight for navigation at night and confuse artificial light with our natural satellite. An international team of researchers challenged this thesis. Their research, which has yet to be peer-reviewed, shows that artificial light interferes with insect flight control systems.
An international team of scientists decided to use a camera to carefully track the movement of insects flying towards the light. Playing the video in slow motion gave them the opportunity to collect information very precisely. The results were recently published on the ‘bioRxiv’ online platform, which makes articles available before peer review.
Types of insect behavior
Three types of behavior have caught the attention of scientists in particular. First, when insects fly over the lights, they often turn around and try to fly upside down, causing them to fall. Secondly, as soon as the insects pass under the light, they begin to loop. When their climb angle becomes too steep, they stop and start to fall. Third, when these creatures approach a light from the side, they can circle or spin around it.
Observations of scientists show that insects fly at right angles to the lights, not directly at them. But since inversions sometimes cause them to fall into the lights, it may appear to the observer that they are flying into the light on purpose.
‘Common to all three behaviors is that the insects turn their backs to light,’ said Samuel T. Fabian of Imperial College London’s Department of Bioengineering. In his opinion, this is evidence that light triggers what is known as the dorsal light response, which orients the animal in such a way that the light is kept perpendicular to the axis of the body. This reflex, which occurs in some fish as well as in many insects, is a way for animals to determine which side an obstacle is on and helps them keep their body upright.
Scientists acknowledge that different animal species rely on the dorsal light response to varying degrees. For example, the oleander moth (Daphnis nerii) and fruit flies did not turn around or orbit near the lights.
Dorsal reaction to light
According to the researchers, in many species the dorsal response to light appears to be programmed. Using a simple computer simulation, they found that virtual insects displaying a dorsal light response also turned, stopped and orbited, just like in the videos used in the study.
The dorsal light response has been known to scientists for decades, but has never been proposed as an explanation for why insects are attracted to artificial light.
In previous considerations, scientists considered that bright light could blind the insects, but that, in turn, did not explain behaviors such as orbiting – that is, spinning around in circles. Many studies have shown that the warmth of the light also does not attract insects.
The authors of the study hope that their findings will help find ways to limit the decline in the number of insects in the world caused by artificial lighting. In their view, one way to protect the insects could be to reduce the number of extremely glaring, unshielded, upward-pointing lights.
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