I got all excited, thinking this was a monarch butterfly, but no, it’s a (still gorgeous) impostor — the caterpillar of the eastern black swallowtail butterfly (Papilio polyxenes). The monarch caterpillar has long “antennae” or tendrils on both ends; the swallowtail lacks these appendages.
The eastern swallowtail is known by a variety of other names, including black swallowtail, American swallowtail, parsnip swallowtail, parsley swallowtail, celeryworm, and caraway worm. It is found throughout southern Canada, the US east of the Rocky Mountains, and south into northern Mexico. It prefers warm, open areas, such as prairies, fields, roadsides, and gardens. They consume plants in the carrot family (Apiaceae), such as mock bishopweed, hemlock, wild parsnip, and cowbane. They can be a minor pest, especially in home gardens, as they also consume related, cultivated plants including caraway, celery, dill, parsley and fennel.
Although they wear warning coloration, like the monarch caterpillar and pipevine swallowtail (which consume poisonous plants and retain the toxins in their body, thereby themselves becoming toxic), Papilio polyxenes is not inherently toxic. They rely partly on their coloration for defense, but if that does not work they can extend two long yellow “horns”, called osmetria, from behind their head, which are coated with a chemical repellent.
These caterpillars are vulnerable to some parasites, some of which locate their hosts by volatile chemicals in the feces. To reduce the chance of being located in this way, eastern black swallowtail caterpillars pick up their feces in their jaws and throw it away!
Learn more about the eastern black swallowtail butterfly here.