Osmeterium

Swallowtail caterpillars have a defense mechanism called osmeterium. Osmeterium are a fleshy organ where caterpillars of swallowtail species sequester essential oils from their host plants, also known as terpenes. These oils coat the normally hidden osmeterium which can be everted or pushed out when the caterpillar is threatened. The osmetierium are usually a bright warning color like red, yellow or orange and because of the terpenes smell quite foul. Having to move swallowtail caterpillars to new host plants also means that butterfly growers get their share of osmeterium love as the caterpillars arch backwards wildly, smearing predators faces and keepers hands alike with gross smelling oils. Each species has its own unique odor based upon what plants the caterpillar has been eating.

The Eastern Black Swallowtail caterpillar shown in these photos has its osmeterium less than half everted, so when fully extended these osmeterium can be pretty showy. This black swallowtail caterpillar was harassed and photographed by the lovely Jill Staake.