Insect "Eyespots" Don't Mimic Eyes, Study Says

Anne Casselman
for National Geographic News
February 22, 2008

The eyespots on the wings of butterflies and moths are intended to be conspicuous to predators, not to resemble the eyes of larger animals, a new study found.

"The story often goes that eyespots on butterflies and moths will startle or intimidate their predators by resembling the eyes of their predator's own enemies," said study lead author Martin Stevens, a behavioral ecologist at University of Cambridge in England.

But with no experimental evidence to support this 150-year-old belief, Stevens and his colleagues put the theory to the test.

Paper Moths

The scientists varied the eyespot markings on artificial paper "moths" and nailed them to trees in the woods of Cambridgeshire, England. They also attached an edible mealworm to each moth to tempt woodland birds, such as blackbirds and house sparrows.

This method allowed them to measure how successful certain patterns were at reducing the likelihood of bird predation, Stevens said.

If eyespots worked by mimicking eyes, then the paper insects with circular spots presented in pairs should be preyed on least.

But this was not the case, Stevens said: Making the spots appear more eyelike by moving the center "pupil" of the eye inward didn't give the paper moths any advantage.

(See a picture of a praying mantis's eyespots.)

Large bars and squares placed on the waterproof paper wings of the experiments provided as much protection as circles—and the larger the marking, the less it was preyed upon.

Likewise, the more the insect spots were on the wing, the less birds attacked them.

(Related: "Hunting Virtual Moths, Blue Jays Offer Eye on Evolution" [February 7, 2002].)

The study appears in the March issue of Behavioral Ecology.

Visual Loudness

The authors concluded that the visual "loudness" of the markings startle or frighten the predator into avoiding spotted prey.

Tom Sherratt, an evolutionary ecologist at Carleton University in Ottawa, Canada, was not involved in the study.

"It does seem very likely, based on [the new] work, that it's the conspicuousness of the signal [that] is more of a deterrent than anything to do with it resembling an eye," he said.

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