You Asked:

Why are insects attracted to light sources? — Ravi, Nagpur, India

Marshall Brain Answers:

On a warm summer night, scenes like this are not uncommon:

Thousands of insects (mostly moths in this case) are attracted to the light. But why are the moths attracted? The funny thing is, no one really knows why.

There are lots of theories though. One of the favorites is the “moon navigation” theory. The idea is that a moth’s simple brain is programmed to keep a bright light positioned at a certain angle on its eye. Presumably, before humans and their artificial lights came along, the only such light at night would be the moon. Since the moon is 280,000 miles away, using it in this way would allow a moth to fly long distances in a relatively straight line.

Today that system does not work well because, when a moth uses it on an artificial light like a porch light or street light, the moth ends up circling the light endlessly.

This theory seems like a reasonable idea. Although it does not explain what moths would do on moonless nights, on cloudy nights, under tree canopies, etc.

See Why are moths attracted to light? and Attraction to light for more ideas.

See also: What’s the difference between moths and butterflies?