Original Article: https://www.nature.com/articles/s41467-024-44785-3
Have you ever wondered why insects always fly into your house when you leave a light on? With records showing light traps being used to trap insects in the Roman era shows just how long this has been known (Ford 1955; Gardiner 1995). Researchers often utilise artificial light to survey insect populations through monitoring the visiting species. This allows for them to better understand what species are present in an area and how many allowing for better conservation efforts. While this attraction is useful for research, it’s not without its negatives, with so many cities constantly being active throughout the night. This has led to a large amount of artificial light pollution being believed to be partially responsible for a decline in insect populations.
There are multiple ideas for why this attraction to light occurs. An example is insects being drawn to light as an escape mechanism. This is due to the artificial light mimicking an area as an escape point from what would be a dark tree canopy or undergrowth. The artificial light source may also mimic the moon which insects would use for navigation causing them to be disoriented. Instead of light, the attraction may be to the heat generated by the lamps. Another theory is that the eyes of the insect, which are adapted to the dark at the time, are suddenly being hit by a bright light causing them to be temporarily blinded and crash (Robinson and Robinson ; Hsiao 1973). The last theory which Fabian and others explore is related to a behaviour known as the dorsal-light-response (DLR) which proposes that an insect will always try and fly with its back (the dorsal side) to the brightest point. This is because as insects have evolved to fly, the brightest point in any environment has always been the sky, meaning that by flying upright it can ensure its orientation is correct. This study seeks to explore this idea further by examining the flight behaviour of flying insects around an artificial light.
Free Flight Experiment
This idea has been tested before but has relied on restricting insects to a set point and not allowing them to fly freely, limiting how well the behavioural response could be measured. To address this limitation and better understand this behaviour Fabian and others performed two separate tests both examining free flight behaviour in wild flying insects at night and examining caught insects in a controlled environment. The behaviour was compared between a single source of intense light (point source light) and light distributed broadly over a wider area more akin to our sky (diffuse light). To track the insects in a controlled environment a small infrared marker was attached meaning that even in a dark environment all movement and direction of the insect can be fully recorded and measured. In the field they were not able to attach the markers so instead relied on the inherent infrared light that the insects would reflect. As insects cannot see infrared light, the light that was used to track them would not affect their behaviour.
What was found?
When exposed to the artificial light the researchers noticed abnormal flying patterns. Of these patterns, three distinct behaviours could be noted. The first was orbiting where the insect would fly in steady circles around the light slightly tilted as to keep its upper side towards the light. The second was stalling where the insect would fly under the light source then turn upwards and fly up with the light behind it, this would lead to the insect slowing down until it would move no more. The last behaviour inversion occurred when the insect flew over the light source to which it would perform a sudden dive before flying underneath the light source to only climb back above it and then invert again. With the diffused light the insect would fly normally if the light remained overhead but when below it would result in the insect crashing into the floor.
These findings followed the theory of DLR with the insect always ensuring their top is aligned with the light. This meant while light was overhead they would fly normally but as soon as light was introduced below the insect they would orient themselves upside down and crash into the floor, as from their perspective the sky had just turned upside down! This suggests that the reason insects often find themselves caught in light traps is not actually due to being attracted to them as people have believed but instead being caught in a loop believing the lamp to be the sky so orientate themselves causing them to circle around it.
There’s more!
As with all things in science (especially animal behaviour!) the results are not always as simple as they seem, with this behaviour differing across different species with some groups of fly for example only showing this behaviour for ultraviolet light. Other groups of insects would not display this behaviour at all. As this was primarily found within large insects, it was suspected that they could rely on passive stability by sensing the forces in flight that comes with their size while smaller insects must instead rely on their sight.
References
Ford, E.B., 1955. Moths (The “New Naturalist” Series). London (see pp. 63–66).
Robinson, H.S. and Robinson, P.J.M., 1950. Some notes on the observed behaviour of Lepidoptera in flight in the vicinity of light-sources together with a description of a light-trap designed to take entomological samples. Entomologist’s Gazette, 1, pp.3-15.
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