The tiger swallowtail is one of the brightest and largest of all North American butterflies. It flies in the early summer, gliding high among the branches of willow, ash, and cherry trees, the food plants of the caterpillar. Like all Swallowtail (Papilio) species, this caterpillar has a defensive mechanism called called an "osmeterium." It's a red, forked gland that it can pop out from behind its head. The organ looks like a snake's tongue and smells like rotten fruit. A very interesting adaptation!