That's why insects congregate around strong light sources – they're 'fooled'

It has long been a mystery why nocturnal flying insects fly around lamps or dive into campfires. There have been many theories. The first is that they are attracted to the warmth of light. Another is that they move by moonlight and thus mistake the outdoor lamp for the moon.

But it has never been possible to support these theories with experiments, says Samuel Fabian, who researches insect behavior at Imperial College London.

He and his colleagues used high-speed cameras to learn how flying insects behave at night around light sources.

He says: – We discovered that they always turn their backs towards the light source.

The trend is showing up

Flying insects have photoreceptors 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 maintain a steady forward flight path.

High-speed cameras It now appears that insects use the same technique when it's dark, but instead it's all the stars in the night sky that show an upward direction.

– They can distinguish between the light of the night sky and the dark earth even when it is cloudy, says Samuel Fabian.

The sensitivity to light of flying insects has helped them distinguish head to head for hundreds of millions of years. But now our artificial lighting confuses insects.

Caught in circles

Their sensory system tells them that they are flying straight ahead when the light is behind them, but in reality they are flying in circles. Unless there is a wind that blows them away from the light, they will remain stuck there.

– They cannot practice their normal habits, such as eating or reproducing, and in the end their energy runs out and they collapse.

Play the video to see what happens when insects get stuck in orbits around our lights.