Turning on outside lights seems to invite every nocturnal insect in the neighborhood to your patio party. Moths and numerous other insects come far and wide to orbit around the light bulbs. I always thought that they had some inbred attraction to light and were intoxicated by it, unwilling to leave until they drop from exhaustion. But according to scientists the bugs are not attracted to light as much as they are disoriented by it.
The most efficient way to get from point A to B is to travel in a straight line. By being able to fly straight, a moth can get to a flower for nectar without burning too much energy, or make it to a receptive female before any of the other guys.
Many flying insects hold a straight line by using light as a compass. When the source is the sun or moon, the light is very distant, and the incoming light rays that reach the insects’ eyes arrive more or less parallel to each other. This is why the sun or moon seems to move with you as you travel, and if you keep the sun at say your one o’clock, you can walk in straight line. Flying insects were designed to receive light at a fixed part of the eye. As long as the moth flies more or less in a straight line, this visual pattern remains unchanged and it is able to hold its course. It’s worked for millennia.
But we humans eventually developed bright light from campfires, candles, and eventually electricity. When a moth flies past this nearby light source, the angle at which the light strikes its eye changes quickly while it holds to a straight-line course. The moth tries to maintain a constant angle to the source, and as it does so, spirals in towards the light and finally gets “stuck” to it, unable to fly anywhere else. So next time a brown beetle flies into your lemonade at an outdoor party, try to remember it’s not entirely his fault. My information source for this article was Earth and Sky, an informative radio show on National Public Radio. Their web site is http://www.earthsky.org.