It has long been a mystery why nocturnal flying insects circle lamps or dive into campfires. There have been many theories. One is that they are drawn to the warmth of light. Another that they navigate by the light of the moon and thus mistake an outdoor lamp for the moon.
– But these theories have never been able to be backed up with experiments, says Samuel Fabian, who researches insect behavior at Imperial College in London.
Together with his colleagues, he has used high-speed cameras to find out how the night-flying insects behave around light sources.
– We discovered that they always turn their backs towards the light source, he says.
Shows the direction upwards
Flying insects have light receptors on their backs. This means that when they fly during the day, they always know that “up” is the direction the light is coming from. With the light on their backs, they know they can keep a steady flight course ahead.
The high-speed cameras now shows that the insects use the same technique when it is dark, but then it is instead all the stars of the night sky that show the direction upwards.
– They can distinguish the difference between the light of the night sky and the dark ground even when it is cloudy, says Samuel Fabian.
The light sensitivity of flying insects has helped them distinguish upside down for hundreds of millions of years. But now our artificial lighting confuses the insects.
Caught in circles
Their sensory system tells them they are flying straight ahead when the light is behind them, but in fact they are flying in circles. Unless there is a wind that blows them away from the light, they will be stuck there.
– They cannot engage in their normal habits such as eating or reproducing and eventually they run out of energy and crash.
Play the video to see how it happens when the insects get stuck in orbits around our lights.