To tell a Two-tailed Swallowtail from a Western Tiger Swallowtail caterpillar, focus on the black edge of the pupil of the large eyespot. For the Two-tailed it is a thin line, while in the Western it should be a thick border. Two-tails turn orange when pre-pupae, which we believe is starting to happen to this individual, where Westerns turn black/brown/purplish. Like many other swallowtails, the younger larvae resemble bird droppings and the older ones have a bright orange, odorous organ (osmetriums) that can be everted from behind the head when under threat.
These caterpillars commonly feed on chokecherry and ornamental green ash in our area. After achieving maximum chunkiness, they overwinter (diapause) as pupae on the base of trunks or on stems. The large and spectacular butterflies will take flight in late spring/early summer. This species ranges across western North America down into Mexico.
Size: Around 3 inches
Photo by: Jennifer Lundberg-DeNeut on 8/28/22 near Clinton, MT