Iridescence is defined as the change in the reflectance depending on the angles of incidence. In the animal kingdom, iridescence is especially widely found among the birds and beetles. The optical mechanisms for iridescence were well elucidated for certain species or lineages with very strong iridescence (such as peacocks and hummingbirds). On the other hand, similar studies are lacking in the species with weak iridescence. Moreover, how and why the colors vary among the related species is less studied. In this study, we aimed to reveal the inter- and intra-specific variation in the mechanisms for iridescence among the flower chafer Protaetia.

We collected the specular reflectance of the elytra of Protaetia species by varying the angle of incidence (θ) from 10 to 55. For the characterization of the internal structure, we examined the cross-sectional TEM images and recorded the number and the thickness of the helicoidal layers of chitin fibers and made inter- and intra-specific comparisons with the the characteristics of iridescence.

In the species we examined, the wavelength where the peak reflectance was observed shifted from 624±80 (mean±SD) nm at θ=10 to 550±69 nm at θ=55. Cross-sectional TEM images reveal the presence of helicoidal layers composed of chitin fiber stacks and the total thickness and the number of helicoidal layers varied greatly. Whereas the average thickness of the helicodal layer showed smaller variation, whether the thickness increases or decreases with the depth of the layer varied. However, we failed to find the structural parameters that can explain the inter- and intraspecific variation in the iridescence in the genus Protaetia. Our results imply a species-specific modulation of helicoidal layers may be responsible for the production of iridescence in this group of beetles.