Experimental degradation of helicoidal photonic nanostructures in scarab beetles (Coleoptera: Scarabaeidae): implications for the identification of circularly polarizing cuticle in the fossil record

Abstract

Scarab beetles (Coleoptera: Scarabaeidae) can exhibit striking colours produced by pigments and/or nanostructures. The latter include helicoidal (Bouligand) structures that can generate circularly polarized light. These have a cryptic evolutionary history in part because fossil examples are unknown. This suggests either a real biological signal, i.e. that Bouligand structures did not evolve until recently, or a taphonomic signal, i.e. that conditions during the fossilization process were not conducive to their preservation. We address this issue by experimentally degrading circularly polarizing cuticle of modern scarab beetles to test the relative roles of decay, maturation and taxonomy in controlling preservation. The results reveal that Bouligand structures have the potential to survive fossilization, but preservation is controlled by taxonomy and the diagenetic history of specimens. Further, cuticle of specific genus (Chrysina) is particularly decay-prone in alkaline conditions; this may relate to the presence of certain compounds, e.g. uric acid, in the cuticle of these taxa.

Keywords: Bouligand structure; Mueller matrix ellipsometry; fossil insects; structural colours; taphonomy.

Figure 1.

Photographs of the five scarab taxa studied, with and without polarizing filters, on untreated, decayed and matured cuticle. Scale in the left-hand column: 20 mm (except G. virens: 10 mm); scale in the right-hand column: 5 mm.

Figure 2.

Light micrographs and scanning electron micrographs of the surface of untreated, decayed and matured elytra. Scale: 50 µm for light micrographs (LM) and 10 µm for scanning electron micrographs (SEM).

Figure 3.

(a–e) Reflectance spectra from untreated (solid black), decayed (red dash) and matured (blue dot) samples of beetle elytra. The spectra were chosen as mostly representative as possible of the global results.

Figure 4.

Mueller matrix 41 obtained on degraded cuticle: (a) from untreated (solid black), decayed (red dash) and matured (blue dot) samples of C. boucardi, (b) from untreated (solid black), decayed (red dash) and matured (blue dot) samples of I. jamesi, (c) from temperature-series maturation of C. boucardi and (d) from temperature-series maturation of I. jamesi.

Figure 5.

SEM micrographs of longitudinal cross sections through untreated, decayed and matured elytra, showing the epicuticle and part of the outer exocuticle. Scale: 2 µm.