This is an earlier instar of the Black Swallowtail caterpillar. Caterpillars go through several stages or "instars" as they molt. The earlier stages can look quite different from the later ones.
Black Swallowtail caterpillar. Also known as the Parsley Worm, because it feeds on parsley and other members of the carrot family.
Black Swallowtail caterpillar, photographed October 4, 2015, at Siebenthaler Fen. The weather was still mild on this date, allowing caterpillars to linger well into fall.
Later instar of a Spicebush Swallowtail. These caterpillars have false eyespots and resemble small green "snakes" to frighten away predators. They usually hide inside a folded leaf for extra protection.
Spicebush Swallowtail. Some caterpillars change colors (for example, from green to brown) when they are getting ready to pupate.
Early instar of a Spicebush Swallowtail. This small caterpillar is a bird-dropping mimic (like the early instar Black Swallowtail above it).
Pipevine Swallowtail. This formidable- looking caterpillar turns into one of our area's most beautiful butterflies. Pipevine Swallowtails ingest toxins from their host plants, and the adult butterflies are poisonous to birds.