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judochop's avatar

What attracts Moth's (and other winged bugs) to porch lights or any light source for that matter?

Asked by judochop (15996 points ) July 29th, 2010

My windows are open in the house and in flew a Moth. He is above me as I type, flying in to the white globe light above my kitchen table. I tried to help it out however he seems content just flying in to the light over and over again and then resting and then repeating said activity. Why? What causes this?
I want to hear your ideas please limit google links and the scientific searches out of this thread we will leave that for the General Section.

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6 Answers

kenmc's avatar

What I’ve read is that moths navigate by sunlight. When they see an unnatural light, they assume it’s the sun and start navigating by it, which leads them to perpetually spin around it.

The exact mechanics, I can’t remember, but I believe that to be the base of it.

Austinlad's avatar

Besides the fact that they don’t make teeny-tiny sunglasses for moths, here’s what I found on Answers.com:

In fact, moths are not attracted to light. they are attrcted to the dark witch is behind the light. I know this sounds wrong. Moths use the sun for navigation, as do many living things. But the sun is, in practical terms, infinitely far away. Light beams from the sun that hit the eyes of a month are all parallel to each other. In fact, all the light beams that hit the earth are parallel to one another.

Moths have been around for much longer than artificial light. When they come close to a light bulb or flame, they are confused. This happens because of the way their eyes have developed, with a more or less complex arrangement designed to pick up parallel beams of light. Moths don’t have the same type of depth perception that we have thanks to the stereoscopic arrangement of our eyes. As a result, they find themselves in a spiral, trying to make sense of what they are perceiving. If it’s a flame, it’s all over for the moth.

ucme's avatar

Well I always thought it was the moon that attracted the little buggers, hence their night time forays. Still, what do I know?

Frenchfry's avatar

I am sure if you were in the dark. you would want to go to the light. They probably want to know what’s going on?

Austinlad's avatar

Mea culpa! Wish I had proof-read and edited the Answer.com text I used above. Goes to prove once again how awful spelling and grammar are on the Web!

Ivan's avatar

@boots and @Austinlad have it mostly correct.

Moths use external light sources for navigation. Before the advent of artificial light, the only light sources a moth would see were stars, the reflection off the moon, and the Sun. These are so far away that, as far as the moth is concerned, they’re infinitely far. That is, no matter how much the moth moves, the position of the light source doesn’t change. This allows the moth to use the light as reference guides for navigation.

Let’s say the moth knows to travel such that the light hits its eye at a 5 degree angle. This is normally fine, but when an artificial light source is added, it presents problems. The moth isn’t “confused”, per se, as it’s just doing what it always does; it just doesn’t understand the difference between a lamp and the sun, because for the millions of years that the moth has been evolving, there haven’t been lamps.

So, the moth continually tries to make the light from the lamp hit its eye at an angle of 5 degrees. Because this angle changes as the moth moves (unlike the light from the sun), the moth has to keep changing its trajectory. If you map this out, you’ll find that the moth’s path forms a spiral, ultimately ending up at the lamp itself.

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