Cell membrane buckling governs early-stage ridge formation in butterfly wing scales

During the development of butterfly wing scales, ordered periodic cell membrane modulations occur at the upper surface of scale-forming cells, priming the formation of ridges. Ridges are critical for wing scale functionality, including structural color, wetting characteristics, and thermal performance. Here, we combine a morphoelastic model based on Föppl-von-Kármán plate theory with experimental observations to shed light on the biomechanical processes governing early-stage ridge formation in Painted Lady butterflies. By comparing the model predictions with time-resolved phase imaging data from live butterflies, we provide evidence that the onset of ridge formation is governed by a mechanical buckling transition induced by the interplay of membrane growth and confinement through association with regularly spaced actin bundles. Beyond ridge formation in Painted Lady scales, our theory offers a rationale for the absence of scale ridges in the lower lamina of many lepidopterans and for the alternating ridge pattern of other butterfly species.