Recent evidence suggests that this species is more appropriately placed in the genus Heraclides. It was formerly treated as Papilio ornythion. This US rarity found only from southern Texas to southern New Mexico is similar to the Giant Swallowtail, but note that the yellow bands in the forewings do not form an "X". Also, note that the tails lack yellow.
Ornythion Swallowtail (Heraclades ornythion), male Chihuahua Woods RR tracks, Hidalgo Co., TX 18 Oct 04
