Caterpillars have some really ridiculous adaptations. There are so many times I look at a caterpillar and think “that’s just silly”, or “is this thing out of its mind?”. And there are always new surprises.

Caterpillar eyespots are one of those ridiculous things. Big cartoonish eyes, staring up at you, daring you to be startled or fooled into thinking it’s really a monster. These eyespots are often accompanied by an enlargement of the body – either solid or inflatable. The can be a variety of shapes, sizes, and colors. Some are simple round dots, while some are oblong and appear to follow you like the eyes in a creepy painting. Some even have little white reflection spots! They can be on either end of the body, hidden or always on display, looking like a snake or who-knows-what. This strategy predominantly occurs in the tropics, among species which hide in rolled leaves.

Some eyespot examples, from Janzen et al. 2010

 The question, of course, is… why? Why did so many species of caterpillars, in a variety of lineages, all evolve different techniques for the same eyespot phenomenon? The answer is most likely: small insectivorous birds.

When you are a bird in the tropics, you have some basic needs. Find food, and stay alive. Caterpillars are delicious, while snakes are dangerous predators. If a bird sees a pair of eyes staring back at them, they have only a split second to decide if it’s worth the risk distinguishing between dinner and becoming dinner. Most often, the choice will be “no thanks, I would rather not disturb that potential predator”, even though the eyes were in fact on the rear end of a caterpillar. It has been suggested that this is an innate response to any startling appearance of eyespots, and not learned by trial and error. So instead of being a normal mimicry complex, the caterpillars are simply finding their own ways to take advantage of this weakness. It does not have to be a perfect match, but enough to make the bird think twice. This occurs mostly in the tropics due to the higher number of small venomous snakes lurking in the vegetation, though we have a few notable species in North America. The spicebush swallowtail has some wonderful eyespots. Perhaps it is relying on the innate responses of migratory birds which spend time in the tropics?

Spicebush swallowtail caterpillar. Image from bugguide.net

 So far most discussions on this topic have had wonderful hypotheses, but are devoid of much data. Some scientists are working toward changing that, and I am excited to see what insights are gained.

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Further reading:
Janzen et al. 2010. A tropical horde of counterfeit predators.
Eisner 2003. For Love of Insects.