I *think* this is an Anise Swallowtail. I was trimming the grass around my vegetable garden when this big fellow climbed up out of the crabgrass and panted at the top of a seed head. I couldn't shut off the trimmer and run inside for my camera fast enough and, while I wish I'd had more time with him to take better pictures, this one turned out okay.
He's obviously yellow and black but he has a little orange spot on each wing and the blue on the bottom wings was incredibly irridescent, as only a butterfly can be. Wingspan of about 4". My Googling found that he doesn't stray far from Western Nebraska and here we are in Eastern Nebraska so he must have hitched a ride to get this far. Or maybe he was resting up from his big trip. Whatever, I'm glad he stopped here.