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Papilio constantinus
Taken at the Chicago Botanic Gardens: Butterflies and Blooms Exhibit. At first I thought this was Yellow Helen (Papilio nephelus), but I'm not so sure.
Beautiful swallowtail!
I don't believe this is a Yellow Helen, the color pattern is different. I would check with the exhibit to find out what butterflies they have on display. I don't believe this is a native species to the US.