A swallowtail caterpillar everts an osmeterium as a defensive organ when threatened.
The osmeterium is a forked, fleshy structure found behind the caterpillar’s head in swallowtail larvae. When disturbed, the caterpillar pushes it outward from a hidden pouch. The structure can release strong-smelling chemicals, including terpenes, that may discourage predators such as birds and insects.
Young swallowtail caterpillars often resemble bird droppings, while older larvae develop species-specific colors and patterns. The osmeterium provides another defense, but it is not a stinger or a venomous appendage. It is generally most effective at close range, when a predator has already approached.
The adult swallowtail has no osmeterium. This is a larval adaptation, just as the adult’s wings and feeding tube are adaptations associated with the later stage of the life cycle. The name comes from Greek roots connected with smell, reflecting the organ’s odor-producing role.