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In butterfly scales, nanostructures that scatter light create structural color, often withvisually delightful effects like iridescence. Structural colors used for signaling, thermalregulation, or camouflage provide excellent case studies in the ecologicalmultifunctionality of color. Developmentally, intricate nanostructures must be preciselysculpted, as a few nanometers’ difference in one dimension would change the hue. Inphysics, the gyroid, an infinitely connected periodic surface once thought to be a purelytheoretical shape, was found reflecting green light in butterfly wings, and naturallyoccurring photonic structures are a rich source of inspiration for optical engineering.Despite interdisciplinary appeal, the biological processes that produce structural colorremain mysterious. Here, I take a telescopic approach to investigate the biology ofbutterfly structural color, beginning with macroevolutionary trends, proceeding togenus- and species-level variation, and concluding with genetic analysis. In chapter one,I aggregate reflectance and morphological data for all 350 described butterfly structuralcolors to interrogate the color gamut and phylogenetic distribution of structural color.In the process, I comprehensively review what is known about how structurally coloredscales develop and evolve. In chapter two, I show that selective breeding shifted wingcolor from brown to blue in buckeye butterflies (Junonia coenia) via a 74% increase in thethickness of each scale lamina. By comparing ten related species in the genus Junonia, Ifind that evolutionarily tuning lamina thickness has generated a wide range ofstructural colors, from gold to magenta and green. A similar thickness increase explainsthe appearance of blue scales in buckeyes with mutations in the optix wing patterninggene. In chapter three, I use a large cross-breeding experiment between blue and brownbuckeyes and quantitative trait locus mapping to identify genetic loci that control theevolved blue structural color. Hue is controlled by a different set of genes than thearrangement of blue scales over the wing surface, and optix is the first specific genefound to regulate the morphology of a photonic structure.