Growing Caterpillar

A black swallowtail butterfly caterpillar. Click to enlarge. 

For the past few weeks, I have been watching black swallowtail caterpillars in a nearby stand of fennel plants. Like many butterfly species, black swallowtails are picky about what they eat. They prefer host plants in the carrot family, Apiaceae, which includes parsley, fennel, dill, Queen Anne's Lace, and of course, carrots.

Fennel smells like licorice candy. It is delightful to look for caterpillars on fennel plants, because as I brush the lacy fronds, they release a sweet licorice smell. I usually wonder in passing what it's like to live on an exclusive diet of fennel.

Adult females lay eggs on the plants. A tiny caterpillar hatches right on its food and starts to eat. It eats and eats until it grows to the maximum size its external skeleton will allow. Then it sits quietly while its skin splits; it emerges larger and ready for the next growth stage. Each stage is called an instar. Black swallowtails have five instars.

Today, as I was taking a walk, I found the caterpillar pictured above. It was sitting very still. I learned later that it was about to change its skin. After walking in the neighborhood for a few hours, I stopped at the fennel plant to look again.  The picture below shows what I saw -- a new instar, feasting on its old skin. The caterpillar was efficiently recycling its resources. I could not help thinking that it probably tasted like licorice.

A newly emerged black swallowtail caterpillar, eating its old skin.