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

The brightest and most vivid colors of butterflies usually originate from light reflecting off the cuticular scales that cover the wing membrane. These scales have an intricate architecture that consists of an upper layer, a grid of longitudinal ridges and transverse crossribs, connected to a lower lamina by pillars called trabeculae. Whereas the role of the lower lamina as a reflector has been well documented in simpler scales, this study unveils the role of the scales upper surface in generating or fine-tuning hue, brightness, and saturation. In the nymphalid Bicyclus anynana, we showed that changes in ridge and trabecula heights accompanied changes in hue of scales produced via artificial selection. We further found that this correlation between ridge height and hue can be generalized to 40 scale types from 35 species across butterfly families. By combining focused ion beam milling, microspectrophotometry, and optical modelling, we found that modifying the ridge height is sufficient to change ridge hue, notably in Morpho didius whose blue color was thought to be generated exclusively by lamella protruding from ridges, rather than ridge height. This study identifies the scales upper surface as a toolbox for structural color diversity in butterflies and proposes a geometrical model to predict color that unifies species with and without Morpho-type Christmas-tree ridges.