Its wings feature an orange and black pattern, and over most of its range it is a Müllerian mimic, meaning it is one of two or more different species with common predators that grow to look alike despite not necessarily being closely related. The other being the monarch butterfly (Danaus plexippus). The viceroy's wingspan is between 53 and 81 mm. It can be distinguished from the monarch by its smaller size and the postmedian black line that runs across the veins on the hindwing. The caterpillar feeds on trees in the willow family Salicaceae, including willows (Salix), and poplars and cottonwoods (Populus).