This is a viceroy, Limenitis archippus. Biology classes often use it as an example of Mullerian mimicry. In this type of mimicry, both species are distasteful to their potential predators. It used to be used as an example of Batesian mimicry, under the assumption that the monarch was distasteful but the viceroy was not.
Then some scientist did an experiment and decided that viceroys were distasteful, so they had to rewrite the textbooks.
I had not seen any viceroys this year until about two or three days ago. Since then I have seen one viceroy on several occasions. Is it the same one or am I seeing different ones? I am not sure.
The roused bear 
And the quick way to tell the difference between viceroys and monarchs is . . . what?
The viceroys have more of a minty aftertaste.
Oh, yeah, they have that black line halfway across the hindwing, too. Monarchs don’t have that.
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