People have long watched moths （飞蛾） and other flying insects flock to streetlamps, lights and flames. These insects appear attracted by the light. But a new study suggests they may just lose track of which way is up.
Previous theories say light probably blinds flying insects so that they get trapped by the light, or maybe they interpret light at night as a place to fly for a quick escape. Now the new study suggests flying insects instead turn their backs to the sky's light to keep their feet pointing toward the ground. Insects naturally turn their backs toward light. But when that light is from an artificial source, it may affect their sense of direction, leading to them flying in circles or diving toward the ground.
At a field station in Costa Rica, Samuel Fabian, an entomologist from Imperial College London in England and his teammates set up hanging and standing lights, and then used high-speed cameras to track wild, flying insects including moths and flies. Some circled the lights endlessly, and others flew sharply upward, losing speed until they couldn't fly any higher. When the light source pointed up, some insects turned around and headed for the ground. During the flight, the insects always kept the lights at their back even if they'd end up crashing. Crash landings were common when the team lit up a white sheet on the floor. But not when a white sheet—stretched into a height above the floor—was bathed in diffuse （漫射） light, much as the sky would be, insects flew through the area without getting trapped by the light.
The team also observed some species in a lab. Moths and dragonflies generally behaved like the wild insects, and they kept the light at their backs. However, in the lab, fruit flies, like oleander hawk moths—which can fly in the dark—could fly over LED lights without being thrown off course. In the wild, though, the moths still crashed. Maybe this is because, Fabian says, the insects can sometimes control their response to light, or over time, they might learn to avoid artificial light.
