Colour and fluorescence emission of Euchroea auripigmenta beetle

In nature, many natural organisms display very conspicuous visual appearances. Most of these appearances are due to pigments located within the biological tissues. In addition, fluorescence emission is also known to arise from several organisms. Upon contact with liquids, the colours and fluorescence of some species such as a few from the class Insecta were reported to change reversibly. However, these optical effects are so far not totally elucidated. In this study, the colour and fluorescence properties of Euchroea auripigmenta beetle were investigated. This insect exhibits a yellow visual appearance on its head, thorax and elytra when it is illuminated by either visible white light or UV light. After soaking into liquids, both the colour and the fluorescence emission from its integuments are modified. The displayed colour turns from yellow to brown. Using optical, fluorescence and electron microscopy techniques, we morphologically characterised the beetle’s integuments. This allowed to observe spike-like protuberances covering the yellow areas of the beetle and from where the yellow visual appearance originates. These protuberances are thought to give rise to further light scattering in addition to the scattering by pigments. Thanks to spectrophotometry, imaging scatterometry and spectrofluorimetry observations, the reflectance and fluorescence properties of this beetle were characterised. Whereas the liquid- induced colour change is attributed to a change in the scattering pattern, the fluorescence emission is most likely due to a chemical influence of the liquids on the two different types of embedded fluorophores.