Which Is It?

This lovely visitor to my flowers set me wondering -- is this a Spicebush Swallowtail or a Black Swallowtail? 

                    Or are there two names for one butterfly?

Fortunately my friend Mr. Google was there, helpful as always.

There are, indeed, two different mostly black swallowtails. And they look an awful lot alike. The Spicebush is larger . . . but that's not a lot of help when there's no Black to compare it with.
Definitive indentification come from a look at the underwing -- Spice (Papilio troilus) has a 'comet' while Black (Papilio polyxenes) has an eye spot.
My pictures don't show the underwing as clearly as I'd like but I think I've identified my visitor. If you'd like to have a go at it, the information on the identifying marks is HERE.. 
And here's a closeup of the relevant area of the underwing. Alas, something has taken a bite out of the tip where the eye spot is (or is not) but isn't that a comet?
As is so often true in this life of ours, it's the little things that make a difference. . .