Morpho butterfly flashiness crucially depends on wing scale curvature.

Morpho butterflies are widely known for their brilliant blue and flashy colours, which are produced by intricate wing scale structures. Not all species display a vibrant structural coloration; some are whitish or even brown. This suggests that there is considerable interspecific variation in wing scale anatomy, pigmentation and flashiness. As evidenced by numerous studies, the optical mechanism that creates the bright structural colours resides in the multilayered ridges of the wing scales, but the interspecific variation in flashiness has so far received little attention. Here, we investigate the wing components that influence the directional wing reflectivity. We therefore selected three species that greatly vary in colour and flashy appearance, Morpho sulkowskyi, M. helenor and M. anaxibia. Applying morphological analyses, (micro-)spectrophotometry and imaging scatterometry on wing pieces and individual wing scales, we demonstrate that wings with flat scales produce highly directional reflections, whereas wings stacked with curved scales scatter light into a wider angular space, resulting in a spatially more diffuse appearance. We thus find that the curvature of the wing scales crucially determines the directionality of Morpho's visual display. We discuss how the visual ecology of Morpho butterflies and environmental conditions can drive the evolution of flashy visual displays.