moths & light

Jim Mason jmason at
Tue May 16 11:26:50 EDT 2000

Curiouser and curiouser!  I confess to being more confused than enlightened
by the responses to date.  Here is another take from the Straight Dope web

This refers to some 20+ year old research (by Dr. Hsiao).  Has this been
revised or significantly challenged since then?

Jim Mason, Naturalist
jmason at
(316) 683-5499 x103
Great Plains Nature Center
6232 E. 29th St. N.
Wichita, KS 67220-2200
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. --Shannon, Montreal

Dear Shannon:

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 came when the moths 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

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. 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. Hey, at some point in our
lives we're all attracted to those bright porch lights. But that doesn't
necessarily you'd want to live above a singles bar.

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