Pale Tiger Swallowtail
I know of no way of distinguishing the immature
stages of this butterfly from those of P. rutulus,
except that the preferred foodplant of eurymedon is Alder.
The taxonomy of the genus Papilio seems curious to a nonspecialist. There are two very different groups - the machaon group of umbellifer-feeding caterpillars, and the tiger swallowtails, whose larvae feed on trees. The two groups are so different from each other that it is hard to know why they are grouped in the same genus. Yet the several species into which each group is split are so similar that one wonders how they deserve species status. While the photographs in this series of adult rutulus and eurymedon seem distinct enough, I have seen butterflies that I could not say which of the two species they are.