Viceroy (Limenitis archippus)
Above: The famous Monarch-mimic, the Viceroy, in Frederick Co., Maryland (9/14/2008). This is the classic example of Batesian mimicry. The harmless Viceroy has evolved to closely resemble the Monarch, whose diet of milkweed makes it toxic to predators. After a wicked Monarch hangover, most predators elect to steer clear of the orange ones, and the harmless Viceroy shares the advantage.
Below: A fresh Viceroy in Prince George's Co., Maryland (6/27/2009).