Several animals, such as birds, butterflies, and beetles, exhibit dazzling colors derived not from pigments but from intricate nanostructures. These structural colors arise from the interference, diffraction, or scattering of light within periodic or quasiperiodic nanostructures. The reflected wavelength changes with the viewing angle, producing brilliant iridescence. Understanding how living organisms control light through their physical structures rather than chemical dyes provides deep insights into the intersection of physics, biology, and evolution.

Among structurally colored animals, jewel beetles are particularly notable. Their metallic luster is produced by a multilayer reflector just beneath the elytron, where alternating cuticular layers with slightly different refractive indices act as natural dielectric mirrors. However, when modeled as a simple periodic stack, the calculated reflectance was surprisingly low. This is because the topmost layer exhibits a lower refractive index, introducing a π-phase inversion in the light reflected at the air–cuticle interface. This phase inversion destructively interferes with the light reflected from deeper layers, thereby reducing the overall brightness.

To resolve this discrepancy, we examined two jewel beetle species, Chrysochroa rajah thailandica and Chrysochroa fulminans, using high-resolution transmission electron microscopy and optical measurements. Our investigation revealed that both species utilize a subtle yet highly effective phase-adjusting mechanism. They modify the thicknesses of the surface layers to compensate for surface phase inversion and recover constructive interference throughout the multilayer system.

In C. rajah, the first low-index layer is notably thicker than the inner layers, which introduces an additional optical path that almost cancels the surface phase shift. In C. fulminans, the adjustment shared by the first and second layers (one low-index and one high-index) indicates that different structural strategies can achieve the same optical optimization.

This phase adjustment mechanism demonstrates how living organisms achieve fine optical control with nanoscale precision. By altering the thickness by only tens of nanometers in the selected layers, these beetles maximize brightness and color saturation. Such subtle structural tuning may have evolved to support visual signaling, such as communication or mate recognition, illustrating how natural selection can lead to photonic optimization. The findings also provide valuable insight for photonic engineering based on natural solutions. Consequently, next-generation structural color materials that achieve high reflectivity with minimal material use may be developed based on our demonstration that the jewel beetle^’s brilliance arises not from simple periodic layering but from precise optical phase design achieved through biological architecture.

(Written by Ryosuke Ohnuki and Shinya Yoshioka on behalf of all the authors)

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