  How pigment distribution correlates with cuticle density within a microscopic
butterfly wing scale, and how both impact final reflected color remains
unknown. We used 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
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, 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.
