We recently received a photograph from a reader in Pennsylvania. The photograph shows a stunning yellow worm-like organism. The creature has plenty of small gray dots along its back, and prominent black eyespots on its head. Here is the picture she shared with us:
We are confident that this is a caterpillar, which means it is the larvae of a moth or butterfly. Sometimes identifying the species of a caterpillar is quite a challenge. This is because markings of two caterpillars of the same species might be very different, while the markings of two caterpillars from different species might appear identical. Luckily for our reader, this is not one of those times! We are confident that this caterpillar is a Papilo troilus larvae, also known as a spicebush swallowtail caterpillar.
Female spicebush swallowtails lay their eggs on spicebush leaves so that when the larvae hatch they can simply begin eating! When they first hatch, spicebush swallowtail caterpillars are brown or yellow (like the one our reader found!), but they turn a greenish yellow color before pupating and maturing into butterflies! These caterpillars hide in folded leaves during the day, and come out at night to feed. If they are disturbed while rolled in a folded leaf, the spicebush swallowtail caterpillar will emit a foul-smelling substance as a defense mechanism to scare off a predator.