Reality is easy. It’s deception that’s the hard work.
~ Lauryn Hill
Nature is full of deception. And now it is being touted by none other than the New York Times. A recent article on caterpillar defenses caught my eye. Researchers studied the late stage larvae of the Canadian Swallowtail (Papilio canadensis). The larvae are plump, green caterpillars with distinctive eye spots that are said to make them snake mimics. This type of mimicry is fairly common in the caterpillar world and is thought to provide some protection from avian predators. When I looked at the photos in the article, they looked very similar to something I found in the yard a couple of weeks ago.
I posted a photo of the third instar larva of an Eastern Tiger Swallowtail (Papilio glaucus) in an earlier post. Like the early instar larvae of many species of swallowtails, it is said to resemble a bird dropping. These so-called bird poop mimics usually have a dark base color and at least one conspicuous white splotch on their body (usually near the center). But, the last two instars of this species look very different.
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?
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.