Swallowtail butterflies are large, colorful butterflies in the family Papilionidae. The family includes the largest butterflies in the world, the birdwing butterflies of the genus Ornithoptera.
Swallowtails have a number of distinctive features; for example, the papilionid caterpillar bears a repugnatorial organ called the osmeterium on its prothorax. The osmeterium normally remains hidden, but when threatened, the larva turns it outward through a transverse dorsal groove by inflating it with fluid. The forked appearance of the swallowtails' hind wings, which can be seen when the butterfly is resting with its wings spread, gave rise to the common name swallowtail. As for its formal name, Linnaeus chose Papilio for the type genus, as Papilio is Latin for 'butterfly'.