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The Straight Dope

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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. --Shannon, Montreal

Always glad to help Straight Dopesters with their homework, ma petite. In fact, considering the number of requests I get, I'm thinking of opening Cecil's Grade-A Term Paper Research Service. Rate: $100 the page. Sure, it's a little steep, but hey, you want to get into med school or don't you?

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 came when they made their sightings from 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 flambe.

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 the ingenious bug researcher 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. When they get close, however, 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, since 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 conjectures that the moth's atom-sized brain figures the darkest part of the sky is safest. Thus it circles around the light (usually at a radius of about one foot, depending on the species) until 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. Call it the "bright porch lights, big city effect": you may want to be where the action is, but you don't necessarily want to live above a singles bar.

You were kind enough to enlighten me concerning pheromones and aphrodisiacs, so I now turn to you again. Can I get AIDS by engaging in sex acts with another woman? Please hurry, my life may depend on your answer. --Robin Razzano, Glendale, California

Last time we talked, Robin, you despaired of ever finding true love. Glad to see you finally found your niche, so to speak. Only one case of female-to-female AIDS transmission has been reported, and that one involved tearing of the skin and mucous membranes. Stay away from false instruments of lust and you should be fine.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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