The physical mechanism of cuticular color in Phelotrupes auratus (Coleoptera, Geotrupidae)

Abstract

The physical mechanism of cuticular color in Phelotrupes auratus was investigated by polarized inspection, spectrophotometry and transmission electron microscopy (TEM). No color change was observed when viewed through either a right-or left-handed circular polarizer. Further, under the incidence of linearly polarized light, the reflected intensity was markedly reduced when observed through a linear polarizer set with its optical axis perpendicular to that of the incident light. These results indicate that P. auratus does not possess any circularly polarizing reflectors. TEM observations revealed a total of ten or twelve thin layers (about 60-120 nm in thickness) of two types of material (electron-dense and electron-lucent) alternately stacked in the epicuticle. The thickness of the layers in the different color forms of the beetle corresponded to the peak wavelengths in the reflectance spectra, lmax(a), with thicker layers found in beetles exhibiting reflectance peaks at longer wavelengths and vice versa. Based on these findings, we concluded that all the cuticular color forms of P. auratus were not produced by a circularly polarizing reflector but by a simple multilayer reflector.

Population genetic structure underlying the geographic variation in beetle structural colour with multiple transition zones

“…The epicuticle of P. auratus elytra is formed by alternative stacking of two types of layers with electron‐dense and electron‐lucent materials, and the elytral coloration is a structural colour based on multilayer reflectors, depending on the density of the layered structure (Akamine, Ishikawa, et al, 2011). The colour variation among individuals within populations in terms of the peak wavelength λmax(α) indicates that coloration is under the control of quantitative trait loci affecting the layered structure, such as the thickness of the different layers, and ultimately, the peak wavelength (Akamine, Ishikawa, et al, 2011). Note that there seems to be little environmental variation in the coloration of P. auratus as we have not noticed temporal variation in colour at the same localities and the present study clarified that colour differences existed despite the similar climatic conditions in the study area.…”

Exoskeleton may influence the internal body temperatures of Neotropical dung beetles (Col. Scarabaeinae)

“…Available studies in coleoptera ( Kinoshita, 2008 ), and specifically in dung beetles ( Pye, 2010 ; Akamine et al, 2011 ), show that color variations are not pigmentary, but structural. Thus, beetle’s color is understood as a pure physical mechanism due to material properties of the exoskeleton which reflects, scatters and deflects the light.…”

Beetle Exoskeleton May Facilitate Body Heat Acting Differentially across the Electromagnetic Spectrum

“…However, the association between immunity and cuticle darkness would not take place in Coleoptera because their color variations are not pigmentary (Kinoshita 2008) but structural (i.e., due to the electron excitation by the incident light). The available studies in Geotrupidae dung beetles indicate that color variations are due to simple multilayer reflectors (Pye 2010;Akamine et al 2011), and, as in other insect groups (Digby 1955;Umbers et al 2013;Välimäki et al 2015), there is no empirical evidence that supports the role of color on heating differences and thermoregulation.…”