ISSUE #27: Bugs Don’t Think Streetlamps Are the Moon
We might have figured out why insects fly around artificial lights at night.
Odds are anyone reading this has seen it before: You’re walking down the street at night, or sitting in the stands at a sports tournament, or chilling on your porch, and you see a bug, or two, or ten zooming around any source of artificial light they can find. Nocturnal insects like moths and flies and beetles are irresistibly attracted to streetlamps and porch lights, and while we’ve found this endlessly fascinating since humans were able to generate light at night, we’ve never hit on an exact reason why.
Before electricity was commonplace, when we’d use oil lanterns and candles and bonfires to illuminate the dark, we thought bugs were attracted to the warmth. “Like a moth to a flame” is still a common adage for self destructive behavior, based on the tendency for moths and other flying insects to occasionally burn themselves up fluttering too close to the firelight. While many living things are indeed attracted to sources of heat, including some bugs, the theory doesn’t really hold up. If an insect wanted to get close to a light for its warmth, then it stands to reason that its goal would be to land on it, whether it’s a candle flame or a lightbulb. But that’s not what happens.
When a bug flies around a source of light in an otherwise dark space, it charts an elliptical path, flying in loops up and down and around the light instead of directly towards it. For a long time, this led many to believe that insects, like some migratory birds, used the light from the moon and stars to navigate at night. The moon and stars move across the sky, but only by increments because of how far away they are. When you walk or drive around after dark, the moon stays in pretty much the same place in the sky, allowing you to orient your position by the direction of its light. Entomologists thought bugs did something similar, so that when they encountered a streetlamp and flew past it, thinking it was the moon, they would have to turn, and keep turning, and keep turning, to keep the source of light in the position they thought it should be.
Except that’s also not what they’re doing. According to new research published in Nature Communications last month, nocturnal flying insects aren’t making constant circuits around random artificial lights because of some complex navigation system. The truth, perhaps, is much simpler: They just think the light is the sky.
By filming a number of flying insects both in a laboratory and in the field in a bunch of different locations, these researchers discovered that the insects tended to turn their backs to the light, instead of flying towards it. This is called a dorsal light response, the ability for an insect or any other motile creature to orient themselves based on the location of the sky and the location of the ground. The sky, usually, is the brightest “direction” no matter what time it is, so to fly straight and horizontal with the ground, all an insect would have to do would be to keep their back to the light. When the insects that the researchers studied flew around and around a brightly shining artificial light, what they were actually doing was trying to keep their back turned toward it, thinking it was the sky, and trapping themselves in perpetual erratic loops.
These light traps can be exhausting for an insect after a while, and many who get caught flying in constant circles around an outdoor lamp will die before sunrise can reorient their senses. I’ve mentioned the perils of light pollution on Bugstack before, explaining how fireflies can confuse faraway lamps or windows for potential mates and waste time and energy trying to find them. For other bugs, artificial nighttime lights are a more immediate death trap, killing by confusion. Bright lights at night are also known to have detrimental effects on humans, throwing off our circadian rhythm and making us groggy and depressed. The darker we keep our streets at night, the better it is for our bugs, and for us.
IN BUG NEWS:
They did surgery on a jumping stick. When a Peruvian jumping stick (a sort of combination of stick bug and grasshopper) molted, one of her keepers at the Houston Zoo noticed a weird dent in her neck that caused her head to flop, and fashioned a tiny neck brace out of a Q-tip and some tape to keep the bug’s head pointed straight up while her new exoskeleton hardened.