Don't touch the parsley worms!

Black swallowtail caterpillars are called parsley worms because they are commonly found on plants in the parsley family, Apiaceae, which includes carrots, fennel, dill, hemlock, Queen Anne’s lace, parsnips, anise, and coriander. I found this caterpillar in the Shakespeare Garden in New York City’s Central Park where it was sharing a fennel plant with  two other parsley worms and a host of wasps and bees.

The black swallowtail caterpillar, Papilio polyxenes.

It is easy to see some features of caterpillar anatomy on a fat three-inch long specimen like this. It has three pairs of tiny true legs near its head, on the right. The four pairs of feet in the middle and the pair at the rear end are called prolegs; they have rows of hooks called crochets on the tips that help them hang on. But one of its most interesting features is usually hidden.

It has an osmeterium, also called a “stink gland,” which it can use to startle and repel would-be attackers. (Other kinds of swallowtail caterpillars have them too.) When I gently tapped this caterpillar, it instantly reared up on its proglegs, stuck out his bright yellow osmeterium, turned to touch me with it, and just as quickly pulled it back inside its slot above the face.

The caterpillar's forked yellow osmeterium is usually hidden. 

The bad smell this left on my finger was impressive. The smell is usually described as that of rancid butter, but it is an angry insect version of rancid butter that seems much worse to me. So I learned a lesson; don’t touch the parsley worms. And if I were a caterpillar predator I would definitely look for a better-smelling meal. 

There is more information about urban insects in my book,  Field Guide to Urban Wildlife, which will be published by Stackpole Books in spring 2011.