mirror determines the far posterior domain in butterfly wings.

Insect wings, a key innovation that contributed to the explosive diversification of insects, are recognized for their remarkable variation and many splendid adaptations. Classical morphological work subdivides insect wings into several distinct domains along the anteroposterior (AP) axis, each of which can evolve relatively independently to produce the myriad forms we see in nature. Important insights into AP subdivision of insect wings come from work in Drosophila melanogaster; however, they do not fully explain the diversity of AP domains observed across broad-winged insects. Here, we show that the transcription factor mirror acts as a selector gene to differentiate a far posterior domain in the butterfly wing, classically defined as the vannus, and has effects on wing shape, scale morphology, and color pattern. Our results support models of how selector genes may facilitate evolutionarily individuation of distinct AP domains in insect wings outside of Drosophila and suggest that the D. melanogaster wing blade has been reduced to represent only a portion of the archetypal insect wing.