Light on the moth-eye corneal nipple array of butterflies

Abstract

The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids.

The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly

“…ARTICLE It explicitly considers the random height distribution of the nanopillars. We start with the calculation of the effective volume fraction f(z) of the wing material similar to the previous studies of gradient refractive index anti-reflective structures [20][21][22] . The subsequent application of the effective medium theory 23,24 allows the calculation of the refractive index profile of the random nanopillars.…”

“…The subsequent application of the effective medium theory 23,24 allows the calculation of the refractive index profile of the random nanopillars. Afterwards, the transfer matrix method (also known as the multilayer model) 20,22,24,25 enables us to compute the reflection and transmission for a given polarization and for all angles of incidence.…”

Bio‐Inspired, Smart, Multiscale Interfacial Materials

“…However, recently some less complex, but nonetheless special features in biology received intense attention (Fig. 2), such as, the selfcleaning effect of lotus leaves and duck feathers, [34,35] the non-fogging, superhydrophobic compound eyes of mosquitoes, [36] the locomotion of geckos and octopuses via highly adhesive feet and suckers, [37,38] the non-wetting phenomenon of water striders walking on water, [39] the color of peacock feathers, butterfly wings, and beetle shells which is caused by a periodic microstructure, [40][41][42] the special nanostructures causing anti-reflectivity in cicada's wings and moth's compound eyes, [43,44] and lastly the special photonic reflectivity of sponge spurs due to their unique microstructure. [45] All these features are suitable for bio-inspiration.…”

“…[46] Nanostructures found in the wings of cicadas and the compound eyes of moths (SEM images in Fig. 2d) can minimize reflectivity over a broad range of angles and frequencies, [43,44] throughgraduallymatchingtheopticalimpedanceofonemedium with its neighbor across the interface. This is achieved through the integration of arrays of tapered elements, so called nipple arrays, into the boundary.…”

Suppression of reflection peaks caused by moth-eye-type nanostructures for antireflection applications studied by using FDTD simulation

“…In this case, the bottom width was w 0 1 + w 2 0 /h 2 , and the width at the top of the nanostructures was w 2 0 /h. Similar parabolic shapes were adopted in other simulation studies [8,12]. In one simulation run, total height h was fixed to a certain value.…”