Nanoscale cuticle mass density variations influenced by pigmentation in butterfly wing scales

TL;DR

This study uses advanced X-ray imaging to reveal how pigment distribution affects nanoscale cuticle density and structure in butterfly wing scales, influencing their reflected color.

Contribution

It provides the first quantitative analysis linking pigmentation to cuticle density variations and structural differences in butterfly wing scales using ptychographic X-ray computed tomography.

Findings

Lower lamina density inversely correlates with pigmentation.

Pigment levels influence chitin structure from sheet-like to rod-like.

Layered density variations explain the reflected color in scales.

Abstract

How pigment distribution influences the cuticle density within a microscopic butterfly wing scale, and how both impact each scale's final reflected color, remains unknown. We use ptychographic X-ray computed tomography to quantitatively determine, at nanoscale resolutions, the three-dimensional mass density of scales with pigmentation differences. By comparing cuticle densities between two pairs of scales with pigmentation differences, we determine that the density of the lower lamina is inversely correlated with pigmentation. In the upper lamina structure of Junonia orithya and Bicyclus anynana, low pigment levels also correlate with sheet-like chitin structures as opposed to rod-like structures. Within each scale, we determine that the lower lamina in all scales has the highest density, and distinct layers within the lower lamina help explain reflected color. We hypothesize that pigments, in addition to absorbing specific wavelengths, can affect cuticle polymerization, density, and refractive index, thereby impacting reflected wavelengths that produce colors.