The Viceroy, Limenitis archippus is one of the most famous examples of mimicry. It resembles the Monarch, Danaus plexippus, which is unpalatable due to cardiac glycosides sequestered from milkweed, its larval host plant. Viceroy larvae, which feed on willows and poplars, do not contain cardiac glycosides and are therefore likely to be more palatable. While predators may have a hard time distinguishing Viceroys and Monarchs by sight, it's easy for humans to tell them apart. The black median line across the hindwing is diagnostic for the Viceroy.