The two-tailed swallowtail is a large swallowtail of western North America, one of several species that have yellow wings with black tiger striping. Each hindwing has several blue markings (top and bottom). Like other striped swallowtails, it has a small orange eyespot near the lower angle of each hindwing; the eyespots can fool predators into attacking the rear of the butterfly instead of the head, giving the butterfly a chance to escape. Its appearance is similar to the western, and eastern tiger swallowtails, but has narrower black stripes and usually two tails on each hindwing (rather than only one). Most two-tails have a black "slit pupil" in the orange eyespot on each hindwing, never present in western tiger. It is also usually larger than similar swallowtails within its range, although some eastern tiger swallowtails are as big as any two-tailed. The wingspan ranges from 3 to 6.5 inches (7.6 to 16.5 cm), making it the largest swallowtail in western North America. As is the case with most swallowtails, females are larger and more brightly colored than males, having more blue in a submarginal band around orange spots on the hindwing upperside, and a deeper yellow background color that can shade to orange. Females also have wider black stripes on the forewings than males. Males may engage in mud-puddling, and have claspers that can be seen as a yellow segment at the end of the abdomen, beyond the black abdominal stripes.