Abstract Vivid structural colors in butterflies are caused by photonic nanostructures scattering light. Structural colors evolved for numerous biological signaling functions and have technological applications. Optically, such structures are well understood, however their development in vivo remains obscure. We show that actin is intimately involved in structural color formation in the butterfly Heliconius sara . Using comparisons between iridescent (structurally colored) and non-iridescent scales in adult and developing H. sara , we show that iridescent scales have more densely packed actin bundles leading to an increased density of reflective ridges. Super-resolution microscopy revealed that actin is repeatedly re-arranged in later development, when optical nanostructures are forming. Furthermore, actin perturbation experiments at these later developmental stages resulted in near total loss of structural color. Overall, this shows that actin plays vital templating roles during structural color formation in butterfly scales, with mechanisms potentially universal across lepidoptera. Teaser The actin cytoskeleton is essential for templating the optical nanostructures responsible for structural color production in butterfly scales. Actin templates the reflective ridges on butterfly scales and is directly involved in forming the color-producing nanostructures within these

The actin cytoskeleton plays multiple roles in structural color formation in butterfly wing scales