The Eastern Black Swallowtail doesn't come around often.  When it does, I sometimes see it laying eggs on my rue plants, where I have found mature caterpillars twice.  The male has a band of yellow patches stretching from wing tip to wing tip above, although the bands appear whitish in one pictured here.  The female's blue patches are more well-defined than the male's, and also helps to distinguish it from the Spicebush Swallowtail.  Below, it is the unbroken row of orange spots which allows one to separate these two species of swallowtail.