All swallowtail larvae have an eversible horn-like organ behind the head known as the osmeterium (osmeteria, plural) that looks like a forked snake tongue. It is a bright yellow-orange color on the black swallowtail. When the caterpillar is disturbed it rears up and the organ is extended for a short period of time. When everted it it releases a chemical repellent with a foul smell to repel predators. It is harmless to humans, however.