Viceroys are smaller then monarchs but the real give away is the black line running through the hind wing, which monarchs do not have. It is believed that the similar coloration helps to fool some predators that avoid eating monarchs because they are toxic because of the milkweed that they eat as a caterpillar. Viceroy caterpillars do not eat milkweed, they mostly feed on tree leaves such as willow, aspen, apple, and cherry, so they are not toxic but it is not necessary to be toxic if you look like another butterfly that is.
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