Several times I have seen Pipevine Swallowtails passing through our yard in Saint Charles, IL, and I have one record for one here at Fermilab.  But the only nearby permanent population of these is thought to be at Waterfall Glen forest preserve, near Argonne National Lab, where I took this photo.  Pipevine Swallowtails are more common further south of Chicago. 

(underside of wings) 

Same butterfly as above in the same location, just a slightly different position.  The blue iridescence results from diffraction of light from the wing scales.