Why Flying Insects Gather at Artificial Light

Why do flying insects tend to circle around a lightbulb or candle? The most popular answer has been that they are trying to hold a constant heading by keeping a constant proportion of the light falling on each eye from a distant source. However, researchers using high-speed video to capture insect flight around various artificial light sources found that the insects being monitored were in fact trying to maintain a level flight attitude by keeping the light source directly above them. If flying at the same level as the light source, turning its back toward the source would necessitate the insect rolling its long axis, causing it to turn and thus circle the light. If flying below the level of the light, the insect was compelled to angle upward until it stalled. If flying above the light, it turned upside down, sometimes crashing into the ground. It’s an example of a control system “designed” to orient the insect’s back toward the sky but yielding maladaptive behavior when confronted with a close source of artificial light.

Great find!