I checked on the swallowtail caterpillar in my yard a week after that first photo and it looked almost exactly like the snake mimic photo of the Canadian Swallowtail larva. Gone was the bird dropping costume and in its place was a light green body with fairly prominent fake eye spots, and a slightly swollen anterior portion, giving it the appearance of a snake’s head.

When the caterpillar is disturbed (I blew on it and touched the leaf for the photo above), it arches its body making the eye spots become more visible and enlarging the “snake head” region. In the recent study reported in The New York Times, researchers studied the predator deterrent efficacy of this snake look-alike strategy by creating tasty plain green “pastry caterpillars”. When placed on twigs, birds readily ate them. They then added eye spots and snake-like heads, and the birds tended to avoid them. In fact, adding just eye spots caused about the same rate of avoidance. This caused them to wonder….If real caterpillars don’t gain extra protection from extra deception, then how could their disguise have evolved?

They decided to present their pastry larvae to day-old bird chicks who had never seen a snake, theorizing that birds may have an innate fear of snakes and snake-like objects. Here are the results…When the chicks were offered simple green cylinders, they grabbed them. But when eye spots were added, the chicks became wary. Consistent with earlier research, the scientists found that adding a snakelike head didn’t make the chicks any more fearful. Over the next two days, the scientists presented the chicks with pastry caterpillars five more times. By the end, the chicks had learned that cylinders with eye spots were, in fact, tasty snacks. That was not the case when the chicks were presented with pastry caterpillars with both eye spots and snakelike heads. Even at the end of the study, the chicks were still fairly wary of the more realistic mimics. They concluded that birds have evolved a fear of snakes and that just a few characteristics, like eye spots, can make them wary. But, birds and other animals, can learn to distinguish between similar-looking objects, which may have driven caterpillars to evolve more elaborate disguises in order to keep fooling the birds.

And, if you are going to have a snake’s head, you may as well have a snake’s tongue. Turns out, all members of the family Papilionidae have a special glandular organ, called the osmeterium, that can be extruded from just behind their head capsule when they are threatened. The forked shape resembles the forked tongue of a snake, further reinforcing this disguise to any would-be bird doubters. The osmeterium also produces secretions with a distinctive and disagreeable odor. These secretions have been shown to repel many invertebrate predators such as ants and mantids, but are not believed as effective against most vertebrate predators.

So, it seems this group of caterpillars are quite adept at the art of deception. From bird poop to strike poses, they offer an array of distasteful, or possibly even dangerous-looking, meal options for foraging birds. I am always fascinated by this long, slow dance of predator and prey and how it plays out in terms of behavior and appearance.