Luke Sullivan, of Oxford Area School, asks :-

Why do moths go to the lights at night?

Anthony Harris, a zoologist at the Otago Museum, responded.

The flight of moths towards street lamps is a problem of the moth using a nearby source of light to navigate by when it is adapted for navigation using only distant sources of light. The moth is not actually attracted to the light.

Moths normally fly in straight lines by using a distant light source such as the moon or a bright star. Moths orient themselves obliquely or at right angles to a source of light in such a way that the compound eye on one side of their head is illuminated more strongly than the other eye. This difference between the amount of light entering the two compound eyes is kept constant. This is called 'Light compass' orientation. If the light source is distant, for example the moon, the moth will fly in a straight line.

However if a street light is the brightest source the moth tries to navigate by it instead. This means the moth will be caused to turn as soon as the relative amount of light entering the two compound eyes changes. The moth adjusts its flight to try to keep these constant. But if the light is nearby the relative amounts of light in each eye can be kept constant only if the moth continually turns towards the source. The moth will thus move along a spiral ending at the light itself.