A meta-analysis of butterfly structural colors: their color range, distribution, and biological production

Butterfly scales are among the richest natural sources of optical
nanostructures, which produce structural color and iridescence. Several
recurring nanostructure types have been described, such as ridge
multilayers, gyroids, and lower lamina thin films. While the optical
mechanisms of these nanostructure classes are known, their phylogenetic
distributions and functional ranges have not been described in detail. In
this Review, we examine a century of research on the biological production
of structural colors, including their evolution, development, and genetic
regulation. We also create a database of more than 300 optical
nanostructures in butterflies and conduct a meta-analysis of the color
range, abundance, and phylogenetic distribution of each nanostructure
class. Butterfly structural colors are ubiquitous in short wavelengths but
extremely rare in long wavelengths, especially red. In particular, blue
wavelengths (around 450 nm) occur in more clades and are produced by more
kinds of nanostructures than other hues. Nanostructure categories differ
in prevalence, phylogenetic distribution, color range, and brightness. For
example, lamina thin films are the least bright; perforated lumen
multilayers occur most often but are almost entirely restricted to the
family Lycaenidae; and 3D photonic crystals, including gyroids, have the
narrowest wavelength range (from about 450 to 550 nm). We discuss the
implications of these patterns in terms of nanostructure evolution,
physical constraint, and relationships to pigmentary color. Finally, we
highlight opportunities for future research, such as analyses of subadult
and Hesperid structural colors and the identification of genes that
directly build the nanostructures, with relevance for biomimetic
engineering.