Help Identify this Swallowtail

I spotted this striped butterfly perched on a pine tree in my yard. It looks like a tiger swallowtail, but I didn't think these butterflies were in this part of the country. Can you tell me what kind it is?

—Jeanne Hardy, Paulden, Arizona

Tom: The butterfly is a female two-tailed swallowtail. This butterfly, which occurs throughout the western United States, is very similar to the common tiger swallowtail, but can be easily distinguished by the double tails on its hind wings. You can tell this one is a female because it has a large amount of blue on the hind wing. This is also a good way to identify a female tiger swallowtail.