Spicebush Swallowtail Caterpillar

Evolution drives innovation and the incredible diversity of caterpillars affirms this. I’ve written about our remarkable caterpillars before, but so many interesting ones inhabit our hills that another look is warranted.

Caterpillars have evolved to look and behave the way they do, largely because of predation. Songbirds, with their keen eyes, are major caterpillar predators, so survival of the fittest favours caterpillars that can fool birds.

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A wonderful caterpillar that lives in southern Ontario is the spicebush swallowtail. Like many caterpillars it has an array of defences but is best known for its resemblance to a snake. It displays false eye spots – “snake eyes” at the front of its snake-like body. The caterpillar’s ruse is enhanced by behaviour. Prod one and it will rear up in a menacing snake-like posture.

Of interest is that none of the snake species that live in Ontario are predators of adult birds. Most of our snakes hunt on the ground and only one or two species climb trees to feed on nestling birds. So why would the spicebush caterpillar’s snake mimicry work?

To find the answer we need to visit the tropics, where snakes, such as the eyelash vipers of Central and South America, wrap themselves around tree limbs to ambush unwary birds.

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Spicebush swallowtail caterpillar just prior to forming its chrysalis. Photo by Don Scallen.

So a fascinating aspect then of the spicebush swallowtail’s fakery is that it mimics snakes that live thousands of kilometres away. Why does this work? Because most of our songbirds spend about half the year in the south. When they migrate to Canada in the spring they pack a menacing visual memory of a tropical threat.

Other caterpillars mimic more ordinary things – leaves for example. I’ve included images of two caterpillars that have evolved to mimic dead or dying portions of leaves. Not as exotic as a snake ruse, but awe-inspiring nonetheless!