It is an early instar of a Black Swallowtail butterfly caterpillar. It will eat and grow until it forms a chrysalis, and then hang out somewhere til spring. Then the butterfly will emerge and look for a partner. They like carrots, fennel, parsley, dill, and rue, and a native plant Alexander something (I can't remember the name). They turn beautiful colored stripes as they get bigger. You'd enjoy raising it, if you have time.

Martha

A beautiful caterpillar, and a beautiful photo.