From the Viette's Views Gardening Blog
Parsleyworms are the larvae of the beautiful Black Swallowtail butterflies which are important pollinators in the garden.
These caterpillars are quite fascinating and very attractive creatures. They go through five larval stages or instars before they pupate and eventually become the beautiful Black Swallowtail butterfly!
The youngest caterpillar, the first instar, is mostly black with small orange spots and a stripe in the center of its body. Each time they molt and progress to the next instar, the caterpillars get bigger and change a little bit. The last instars are the really cool, colorful ones. They get up to 2″ long.
These voracious eaters can devour all the leaves on a parsley plant in no time, leaving just the bare stems, but don't kill them! In order to have butterflies, you must be willing to allow some plants to be eaten by the caterpillars! These guys also like Queen Anne’s Lace so rather than kill them when they get in your garden, just transport them all to a wildflower garden and they’ll be just as happy.
One really interesting thing about parsleyworms is that when they are disturbed or threatened, they rear up and protrude a pair of fleshy, orange “horns” just behind their head. This is a gland called an osmeterium which emits a very foul smell to deter would be predators.
As gardeners, we are always trying to attract the beautiful butterflies into our gardens and one of the best ways to do this is to plant some host species for them to lay their eggs on. These host plants will provide “food” for the caterpillars when they hatch.
Black Swallowtail caterpillars (the parsleyworms) specialize in eating plants in the carrot family like parsley, celery, dill, and fennel – all things that we might grow in our gardens. So plant a few extra seeds throughout your perennial gardens to feed these colorful, hungry caterpillars. You’ll be rewarded with beautiful butterflies flitting and frolicking through your gardens happily feeding on nectar and pollinating all your flowers!