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Why is that caterpillar looking at me?

On a trip to Quartz Lake, visitor to Alaska Garrett Ast once plucked a caterpillar from a twig.

As Garrett held it in his palm, the caterpillar reared up and — with two sparkling baby blues — looked him right in the eye. Upon closer inspection, my nephew saw that, though striking, the caterpillar’s eyes weren’t real.

So was born the question of why a caterpillar might invest energy in producing a set of fake eyes. A little investigation led to a science research paper with a fine example of a journalistic lede:

“You are a 12-gram, insectivorous, tropical rainforest bird, foraging in shady, tangled, dappled, rustling foliage where edible caterpillars and other insects are likely to shelter. You want to live 10-20 years. You are peering under leaves, poking into rolled ones, searching around stems, exploring bark crevices and other insect hiding places. Abruptly an eye appears, 1-5 centimeters from your bill.

“If you pause a millisecond to ask whether that eye belongs to acceptable prey or to a predator, you are likely to be — and it takes only once — someone’s breakfast. Your innate reaction to the eye must be instant flight.”

John Burns of the Smithsonian Institution helped craft that sentence as one of three authors of “A tropical horde of counterfeit predator eyes,” which appeared in Proceedings of the National Academy of Sciences in 2010.

Burns and his co-authors posted excellent photos of more than two dozen tropical caterpillars with elaborate false eyes. The caterpillars probably evolved those false eyes to mimic snakes, lizards, small mammals and other things that eat little birds.

But wait a second — there are no snakes or lizards in Alaska. Why would an Alaska caterpillar with aspirations of turning into a swallowtail butterfly pose as a reptile?

I sent the photo to Burns, and to Derek Sikes, curator of entomology at the University of Alaska Museum of the North.

“Birds learn about snakes when they migrate (to the tropics and other places warm enough for snakes),” Sikes wrote in an email. “So, the snakes don’t have to be here for the mimicry to work. Nice, eh?”

Burns said even the rugged birds that don’t flee Alaska for the winter might have the image of a snake wired deep within their tiny brains, even though they will never encounter one.

“Despite the lack of snakes in Alaska, a small insectivorous bird might still be genetically programmed to retreat when abruptly confronted at close range by the caterpillar’s ‘eyes,’ owing to the bird’s evolutionary ancestry,” Burns wrote.

“A resident bird species (like a chickadee or redpoll) might have descended in the not-too-distant past from a species that spends much of its life in a tropical environment, where selection would directly preserve such behavior.”

Since the late 1970s, the ��������' Geophysical Institute has provided this column free in cooperation with the �������� research community. Ned Rozell is a science writer for the Geophysical Institute. A version of this article was first published in 2010.