The mechanisms underlying the evolution of morphology are poorly understood(1,2). Distantly related taxa sometimes exhibit correlations between morphological differences and patterns of gene expression(3-8), but such comparisons cannot establish how mechanisms evolve to generate diverse morphologies. Answers to these questions require resolution of the nature of developmental evolution within and between closely related species. Here I show how the detailed regulation of the Hox gene Ultrabithorax patterns trichomes on the posterior femur of the second leg in Drosophila melanogaster, and that evolution of Ultrabithorax has contributed to divergence of this feature among closely related species, The cis-regulatory regions of Ultrabithorax, and not the protein itself, appear to have evolved. This study provides experimental evidence that cis-regulatory evolution is one way in which conserved proteins have promoted morphological diversity(1).