Bright Silver Brilliancy from Irregular Microstructures in Butterfly Curetis acuta Moore

Abstract

Structural whiteness in nature is one of the useful optical phenomena contributed by refined microstructures and serves important biological functions. However, whiteness residing in wings, antennae, and bodies of butterflies often draws less attention, with undetected physical mechanisms and applications. Here, it is discovered that bright silver scales on the ventral side of butterfly Curetis acuta Moore have enhanced broadband reflection with particular angle dependency, which is dominated by the irregularity of scales. The broadband reflection is the result of color mixing effect caused by laminated scales with irregular layer space and thickness. The special angle dependency suggests that the reflection intensity changes with azimuth angles but remains unvaried at different viewing angles along given direction. This is the consequence of diffraction caused by ridges and irregular holes on the scales, which generates the shining effect and extends the viewing angle of broadband reflection. The characteristic reflection property is speculated to be used for intraspecific communication. Furthermore, it is found that the broadband reflection property of the silver scales is helpful to lower the body temperature under direct sunshine. These findings for light and thermo controlling would offer a potential strategy to design bioinspired advanced materials for low‐energy, reflectance‐based applications.

Convergent Evolution of Broadband Reflectors Underlies Metallic Coloration in Butterflies

“…All three types of metallic scales differed from other sub-families. The silver scales of C. lohita showed optical and structural features reminiscent of the undulatory thin-film configuration found in Nymphalidae, Hesperiidae, and Pieridae in this study, as well as in the specular scales of Curetis acuta, another lycaenid (Wilts et al, 2013;Liu et al, 2019). In particular the reflectance spectra, color speckling at microscopic levels, upper lamina external views, and transversal inner anatomies all resembled the hesperid and pierid samples (Figures 6A,D,G,H,M).…”

“…Those cases invariably display a highly reflective configuration under the form of ectopic lamination, i.e., an apparent covering of the micropores (sometimes called windows or pepper-pots) that perforate the upper lamina of less reflective scales. This kind of simplification of the upper lamina to a film-like sheet has been consistently observed in scanning electron micrographs of silver-metallic scales, including from nymphalid species of the Heliconiinae and Satyrinae subfamilies (Simonsen, 2007;Giraldo, 2008;Vukusic et al, 2008;García-Barros and Meneguz, 2012;Dinwiddie et al, 2014), in the lycaenid Curetis acuta (Wilts et al, 2013;Liu et al, 2019), and in the hesperid Carystoides escalantei (Ge et al, 2017). In the polymorphic species Argynnis niobe, scales from flat white spots show more perforation of the upper lamina compared to scales from individuals with brighter, recognizably silver spots (Simonsen, 2007).…”

Biologically inspired flexible photonic films for efficient passive radiative cooling

“…One of the fascinating phenomena is structural whiteness or silver color, which can be prevalently found in insects (33)(34)(35), aquatic organisms (36,37), and mammals (38,39). Over the past few years, numerous biological creatures have been reported to exhibit a thermal regulation ability (40)(41)(42)(43)(44). For example, silvery Saharan ants were reported to exhibit thermal control behaviors via their triangularshaped hairs, which exhibited high reflectivity in the visible to near-infrared (vis-NIR) region and high emissivity in the midinfrared (MIR) region (40).…”

Springtail coloration at a finer scale: mechanisms behind vibrant collembolan metallic colours

“…colours of the rainbow), although there is increasing interest in the colour production mechanisms of scales with a broadband, metallic reflection (especially in butterflies). These studies reveal the importance of thin-film effects of upper/lower laminae and of the cross-ribs [8,[11][12][13][14][15][16][17]. By contrast, colour production of metallic scales of early Lepidoptera and basal arthropods which have many scaled representatives (e.g.…”