At night, porch lights and other artificial light sources are usually buzzing with flying insects. Why are insects attracted to lights, and why do they keep circling around and around like that?
Turn on your porch light after sunset, and you will be treated to an aerial display by dozens, if not hundreds, of bugs. Artificial lights attract moths, flies, crane flies, mayflies, beetles, and all sorts of other insects. I often find frogs and other insect predators hanging around my porch at night, taking advantage of the easy pickings.
Unfortunately for the insects, their attraction to artificial light is a cruel trick caused by our innovation moving faster than their evolution. Night flying insects evolved to navigate by the light of the moon. By keeping the moon's reflected light at a constant angle, the insects can maintain a steady flight path and a straight course.
Artificial lights interfere with an insect's ability to detect the moonlight. They appear brighter, and radiate their light in multiple directions. Once an insect flies close enough to a light bulb, it attempts to navigate by way of the artificial light, rather than the moon.
Since the light bulb radiates light on all sides, the insect simply cannot keep the light source at a constant angle, as it does with the moon. It attempts to navigate a straight path, but ends up caught in an endless spiral dance around the bulb.
Some scientists believe light pollution is leading to a decline in certain insects. Fireflies, for example, have difficulty identifying the flashes of other fireflies where artificial lights are present.