Contrary to what one might think, insects do not point directly at the artificial light source, but rather tilt their backs towards it, randomly deflecting and changing their trajectory. However, in natural light, similar behavior helps them maintain the correct position in the air. A team of biologists examined it and recorded it with a high-speed camera.
Who hasn’t seen moths and other “bugs” flying around streetlights?
Artificial light has long been known to attract flying insects and has been used to trap them since Roman times. However, the reasons for this behavior are not fully known and various possibilities have been suggested, including: the use of light as an escape route, insects being blinded by the light source, or confusion with the moon… and many others. Theories.
To investigate this mystery, an international team of researchers led by Samuel Fabian from Imperial College London (UK) and Yasha Sondhi from Florida International University (USA) used high-speed cameras. Together they observed the three-dimensional flight of many species of moths, butterflies, dragonflies and fruit flies, both in the laboratory and in the wild at the Monteverde Biological Station in Costa Rica.
Scientists studied its flight under various lighting conditions, including point sources of ultraviolet radiation and on surfaces with more diffuse light. In this way, they confirmed that insects are a “dorsal response to lightand adjust their orbits so that their backs are facing the light source. According to a study published in the journal, this limits your ability to navigate Nature communication.
When natural light sources such as the sun or a starry night are used, this reaction causes insects to maintain constant flight and precise orientation with respect to the horizon. However, artificial light causes its trajectory to change and adjust irregularly, often circling around its focal point.
“The most important finding is that insects confuse light with the upward direction of the sky“, explains Fabián and continues: “To fly, it is important to know where the sky is, as it is necessary to create forces that counteract gravity“. “This confusion then causes the insect to tilt its body toward the light and direct its flight against gravity. This leads to its winding orbit that we often see near streetlights. The most reliable evidence of this is when they fly over a bright light, turn around and fall out of the air.“.
The results also allowed us to rule out some hypotheses that were considered but not tested: “We do not believe that this is a positioning in relation to the moon, and we also observe that the insects do not fly directly towards the light. Rather, it confuses them vertically and traps those who happen to pass by.said the researcher.
Occurs in many different types of insects
The team tested the same behavior on ten different species of insects. “This makes sense because everyone faces the same problem: figuring out where gravity is. When they fly, they cannot use ground reaction forces (like we do when walking), and when they turn corners, they experience various types of accelerations (G-forces), which are often greater than gravity and imperceptible. For example, direct detection by leg suspension is an imprecise mechanism.“, explains Fabián. “But leaning your back toward the light, what you think is the sky, is a good way to maintain an accurate position in the air.“, he emphasizes. This is a reliable and fast mechanism that does not require additional sensors.
“Insects (including other animals such as fish and marine invertebrates) constantly use this response to quickly tilt in the right direction, and this mechanism remains very useful throughout the day.According to the authors, who tested this using diurnal and nocturnal insects, the back’s response to light appears to be both diurnal and nocturnal.
Two exceptions
However, the researchers found two exceptions to this behavior. The first is the fruit fly (Drosophila), which flew through the UV light zone without bending or turning around. The reason for this different behavior is unknown, but researchers around the world are taking advantage of this by using these flies for aerial experiments where light or visual environments do not interfere.
The second exception is the Sphinx oleander (Daphnis nerii), which turned around when exposed to ultraviolet light and showed no changed flight patterns in the laboratory. “This is particularly strange since this species approaches light traps in the field but not in the laboratory, and one reason may be that these moths have different flight modes.said Fabián as he glided along at high speed. “It is clear that insects can sometimes suspend the response to backlight when it suits them, and we wanted to investigate this further.“.
The problem of artificial light and light pollution
In any case, this zoologist mentions that light was a good indicator of insect flight direction for 370 million years, but recently humans have changed it with night lights: “When we started putting big, bright lights everywhere, this became a problem.“.
“Suddenly the brightest area visible at night is no longer the sky, which is extremely rare for insects. However, I would like to emphasize that they are not stupid, but are wonderfully adapted to function in their natural environment. The reality is that we have changed the environment faster than they can adapt to it.“
Fabián emphasized the impact “very negative” of nighttime illumination in insect populations. “This not only attracts and traps them, but also disrupts their activity time. For example, nocturnal animals typically sleep when exposed to light.” “Pesticides and land use changes could have even more devastating effects, but we still don’t know exactly how much damage artificial light causes at night“.
Next step: determine the distance
The authors concluded that more research is needed to determine the effects of artificial light over long distances and that we can improve the habitats of these small animals by reducing unnecessary artificial light at night.
“Our next big question is at what distance this effect occurs with different light sources“says Fabian and comes to the conclusion:”Our current data is around 2 meters around the maximum of the light source, but we don’t know what happens at 20m, 100m or 1km. Understanding this is key to efforts to protect and reduce the impact of light pollution on our nocturnal wildlife.“.
Reference:
Samuel Fabian et al. “Why Flying Insects Gather in Artificial Light.” Nature communication2023.
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