A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why are moths attracted to bright lights?

January 27, 1989

Dear Cecil:

The other day one of my professors asked why moths were attracted to light. Someone thought it might be because they thought it was the moon. But even granting that moths might not be bright enough to tell a porch light and a celestial body apart, why should they be interested in the moon? Please, Cecil, this may be worth extra credit to me.

Cecil replies:

Always glad to help Straight Dopesters with their homework, ma petite. You going to let me have the gold star?

For many years it was thought the moon did have something to do with the attraction of moths to light. The so-called light-compass theory held that moths used the moon as a navigational beacon. By keeping it at a constant angle to their direction of travel, they were supposedly able to fly in a straight line. The trouble (for the moths) came when they made their sightings on a close-up light source like a candle flame. Instead of heading in a straight line, they flew around the flame in an ever-narrowing spiral until finally, phhhht, moth flambé.

But this theory had more holes in it than a moth-eaten sweater. The main problem was that moths simply don't fly around lights in spirals. This was shown by an ingenious bug researcher named Henry Hsiao. He tethered moths to little styrofoam boats in a tiny artificial pond — I love guys like this — and tracked their flight as they headed toward a light source. He found the moths flew more or less straight at the light until they got up close, at which point they veered off and circled around it at a more or less constant distance. They seldom actually touched the light.

A number of other theories have also been discredited. Some claim that, to the moth, bright lights mean open space and open space means safety. But moths are nocturnal, and the night sky has no light sources anywhere near as bright as a porch light. Besides, why should the moth feel compelled to fly around the light in circles? Others argue that moths associate light with warmth. Yet ultraviolet lamps, which are much cooler than incandescent bulbs, attract more moths.

Henry Hsiao to the rescue. He said moths exhibit two kinds of behavior. When they're distant from a light source (they're drawn to light from as far as 200 feet away), they make a beeline straight toward it. Why, nobody knows. Maybe they've tumbled to the fact that lights mean people, and people mean: Wool sweaters! On an even more basic level, a light means: Other moths! Par-ty!

However, when the moths get close to the light, a different kind of behavior takes over. Instead of being attracted to the light, the moth is actually trying to avoid the light. When you think about it, this is only natural. To a creature of the night like a moth, daylight and by extension any bright light means danger. The moth doesn't fly directly away from the light due to a peculiarity of vision called a Mach band. A Mach band, which apparently is common to all sighted creatures, is the region surrounding a bright light that seems darker than any other part of the sky.

Hsiao conjectured that the moth's atom-sized brain figures the darkest part of the sky is safest. So it circles the light in the Mach band region, usually at a radius of about one foot, depending on the species. Eventually either its momentum carries it away or it finds a dark corner to hole up in.

In short, moths like some light but not too much — just like other creatures I could name. Nobody wants to get burned, naturally, but at some point in our lives, aren't we all attracted to those bright porch lights?

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