Thin‐film iridescence in the eggshell of a stick insect ( <i>Myronides glaucus</i> )

Abstract Colours in nature can be pigmentary, structural or a combination of both. The prevalence, function and nanostructural origin of structural coloration in eggs is largely unknown. Stick and leaf insect eggs display a wide variety of colours, most of which are produced by pigments. The eggs of Myronides glaucus (Phasmida: Lonchodidae; Hennemann, 2021), however, show a clear purple to green iridescence. Here, we use micro‐spectrophotometry, Fourier‐transform infrared reflectance, transmission‐ and scanning electron microscopy, atomic force microscopy, finite‐difference time‐domain optical simulations and experimental approaches to elucidate the mechanism for iridescence in M. glaucus eggshells, which together reveal that iridescence is caused by thin‐film interference by a 200‐ to 450‐nm‐thick outermost layer. These results highlight the diversity of phasmid eggs and the need to study the different mechanisms and functions of structural coloration.