Colour changes upon cooling of Lepidoptera scales containing photonic nanoarchitectures

TL;DR

This study investigates how water vapor condensation during cooling affects the color of Lepidoptera wing scales with photonic nanoarchitectures, revealing that open structures change color while closed ones do not, enabling species identification.

Contribution

It demonstrates that cooling-induced water condensation causes color changes in open nanoarchitectures, providing a new method to identify species based on nanoarchitecture openness.

Findings

Open nanoarchitectures show color shifts upon cooling.

Closed structures exhibit minimal or no color change.

Color changes are reversible after warming and drying.

Abstract

The effects produced by the condensation of water vapours from the ambient in the various intricate nanoarchitectures occurring in the wing scales of several Lepidoptera species were investigated by controlled cooling (from room temperature to -5 - -10 {\deg}C) combined with in situ measurement of changes in the reflectance spectra. It was determined that, due to this procedure, all photonic nanoarchitectures giving a reflectance maximum in the visible range and having an open nanostructure exhibited alteration of the position of the reflectance maximum associated with the photonic nanoarchitectures. The photonic nanoarchitectures with a closed structure exhibited little to no alteration in colour. Similarly, control specimens coloured by pigments did not exhibit a colour change under the same conditions. Hence, this effect can be used to identify species with open photonic nanoarchitectures in their scales. For certain species, an almost complete disappearance of the reflectance maximum was found. All specimens recovered their original colours following warming and drying. Cooling experiments using thin copper wires demonstrated that colour alterations could be limited to a millimetre, or below. Dried museum specimens do not exhibit colour changes when cooled in the absence of a heat sink due to the low heat capacity of the wings.