Meeting Abstract
Understanding animal coloration is important for investigating sexual selection, speciation, and animal signaling. Despite a growing number of papers investigating structural colors, the role of nanostructures in creating black color patches has largely been ignored. Recently, it has been shown that certain animals have evolved micro- or nano-structures responsible for creating matte-black surfaces with reflectances approaching the darkest synthetic materials. It has been shown that certain papilionid butterflies reflect as little as 0.2% of incident light, and this phenomenon is mediated by a honeycomb scale structure with melanin bound to the cuticle. It is unknown, however, if other ultra-black butterflies use this mechanism and whether we can derive general principles about the design of ultra-black materials from butterfly scales. We examined butterflies from four subfamilies and demonstrate that ultra-black color can be achieved through various scale geometries from honeycombs to rectangular holes. Using scanning electron microscopy, we found considerable interspecific variation in the geometry of the holes that does not mirror differences in reflectance. Furthermore, we verified with finite-difference time-domain modeling that the two structural features found consistently in ultra-black scales – steep ridges and expanded trabeculae – each reduce reflectance by up to 16-fold compared to scales lacking these features. Our results demonstrate that butterflies have convergently evolved ultra-black scales by creating a material with high internal surface area that minimizes surface reflection and increases the opportunity for absorption. We hypothesize that butterflies use these ultra-black wing patches to increase the perceived brightness of color signals for use in intra- and interspecific signals
