Ridge and crossrib height of butterfly wing scales is a toolbox for structural color diversity

Butterfly wing vivid colors arise from light reflecting off cuticular scales, whose architecture consists of longitudinal ridges and transverse crossribs connected to a lower lamina by trabeculae. While the lower lamina's reflective properties are known in simpler scales, the upper surface's optical characteristics have only been thoroughly investigated in the case of modified ridges. This study examines how the lower lamina and the upper surface contribute to hue, brightness, and saturation across species. In Bicyclus anynana, ridge height changes correlate with hue shifts from artificial selection. This relationship holds true across 39 scale types in 34 species. By combining focused ion beam milling, microspectrophotometry, and optical modeling, we found that modifying ridge height is sufficient to change ridge hue, notably in Morpho didius, whose color was thought to be generated exclusively by ridge lamellae. This study identifies the upper surface of scales as a toolbox for structural color diversity in butterflies and proposes a geometrical model for predicting color that unifies species with and without Morpho-type ridges.