Humidity-dependent colour change in the green forester moth, Adscita statices

Abstract

The colours of insects serve important visual functions in aiding mate recognition, camouflage and warning. The display of insects is usually static, as cuticle coloration does not (or hardly) change during the lifespan of a mature imago form. Here, we describe a case of humidity-dependent, brilliant coloration in the green forester moth, Adscita statices. We show, by employing spectroscopic and ultrastructural methods, that the moth's colour results from the interference of incident light with an unusual hydrophilic melanized-chitin multilayer present in the wing scales. Humidity changes in the environment affect the multilayer properties, causing a significant shift of the green-peaking reflectance in the dry state to a rusty colour when damp, resulting in the strong colour change between day and dusk or dawn.

Keywords: Lepidoptera; body colours; camouflage; colour change; photonic nanostructures.

Figure 1.

The colour of the green forester moth, A. statices, differs between the damp and dry state. (a–c) Habitat photographs (taken by Markus Enekvist) of the green forester moth in various states from damp (a) to dry (c). (d,e) Epi-illumination light micrographs of the scale lattice on the dorsal side of the forewing when dry (d) or damp (e). Note the strong colour change of the scales from green in its dry state to rusty-red when damp. (f) Reflectance spectra of the wing scales in the dry (green, solid line) and states of increasing humidity (orange to red, dotted lines).

Figure 2.

Ultrastructure of the pigment-containing wing scales. (a–c) Scanning electron micrographs. (a) The scale lattice shows two different types of wing scales. (b) Zoom-in on the different scale types shows a porous cover scale (c) and an open ground scale (g). (c) The cover scale shows a porous top surface. (d–f) Transmission electron micrographs show a multilayered arrangement in the cover scale lumen (e), which is absent in ground scales (f). (g) Simulated humidity-dependent reflectance spectra for a melanin-containing chitin structure. The background refractive index n was varied between air (n = 1, solid green line) and water (n = 1.33, dashed-dotted red line) with various humidity levels in-between. (Online version in colour.)