Eastern Black Swallowtail Butterfly Caterpillar

White caterpillar with yellow-and-black markings feeding on fennel. My daughter discovered that when you squeeze it, what look like yellowish-orange horns emerge from its head. She has a good sense of smell and says they emit a scent when they emerge. We found out these are a defense mechanism specific to Swallowtail larvae, and they are called "osmetrium." According to Wikipedia, "This organ emits smelly compounds believed to be pheromones."