This was one of the more interesting butterflies we encountered in the Chisos Basin. After reading up on the various dark swallowtails of the western U.S., I learn that details of the HW eyespot rule out Anise Swallowtail, and the true "Desert" Black Swallowtail reportedly doesn't range as far E as Texas. The large amount of yellow on the HWs here is apparently just a rare color form of the "Eastern" Black Swallowtail, termed form "pseudoamericus" by Scott (1986, p. 172). We saw a fair number of normal ("Eastern") Black Swallowtails during the course of our visit (probably 10 or 20 of them).
This butterfly was first discovered by Mary Kay Sexton. It was particularly fond of Firecrackerbush (Bouvardia ternifolia).