Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)

Abstract

Members of the order Coleoptera are sometimes referred to as 'living jewels', in allusion to the strikingly diverse array of iridescence mechanisms and optical effects that have arisen in beetles. A number of novel and sophisticated reflectance mechanisms have been discovered in recent years, including three-dimensional photonic crystals and quasi-ordered coherent scattering arrays. However, the literature on beetle structural coloration is often redundant and lacks synthesis, with little interchange between the entomological and optical research communities. Here, an overview is provided for all iridescence mechanisms observed in Coleoptera. Types of iridescence are illustrated and classified into three mechanistic groups: multilayer reflectors, three-dimensional photonic crystals and diffraction gratings. Taxonomic and phylogenetic distributions are provided, along with discussion of the putative functions and evolutionary pathways by which iridescence has repeatedly arisen in beetles.

Figure 1

Examples of beetle iridescence. (a) Loxandrus rectus (Carabidae: Harpalinae), (b) Phalacridae gen. sp., (c) Cicindela scutellaris scutellaris (Carabidae: Cicindelinae), (d) Amarygminae gen. sp. (Tenebrionidae), (e) Phanaeus vindex (Scarabaeidae: Phanainae), (f) Eupholus sp. (Curculionidae: Entiminae).

Figure 2

Multilayer reflectors. (a) A schematic of simple cuticular multilayer reflector, (b) Cicindela scutellaris, TEM cross section of cuticular reflector, (c) simple multilayer colour in a buprestid; schematic of (d) epicuticular reflector, (e) exocuticular reflector and (f) endocuticular reflector.

Figure 3

Additive coloration systems in Cicindelinae. (a) Cicindela repanda, habitus view, (b) Cicindela repanda elytral surface with punctae, and (c) close-up view, SEM of elytron showing epicuticular reflector (arrow) and surface microsculpture, (d) reflectance spectra of punctae and surrounding areas (5 m²) of the elytron, the entire elytron and sand substrate (T. D. Schultz 1994, unpublished data). Green triangles, puncta; yellow triangles, perimeter; red triangles, field; open circles, wet sand; filled circles, elytron.

Figure 4

Circularly polarized multilayer reflectors. (a) Schematic of helical multilayer reflector, (b) Chrysina boucardi viewed through quarter wave plate rotated 0°, and (c) C. boucardi viewed through quarter wave plate rotated 90° clockwise.

Figure 5

Broadband multilayer reflectors. (a) Schematic of chirped reflector and (b) broadband reflectance in Chrysina chrysagyrea (Scarabaeidae: Rutelinae).

Figure 6

Phanolinus sp. (Staphylinidae: Staphylininae) specimen exposed to sunlight (including UV) on left side for 20+ years.

Figure 7

Three-dimensional photonic crystals in weevils and longhorn beetles. (a) Pachyrrhynchus congestus pavonius (Curculionidae: Entiminae), SEM of crystal structure from scale interior (from Welch & Vigneron 2007), (b) SEM of the interior structure of Prosopocerus lactator (Cerambycidae: Lamiinae), (c) Pachyrrhynchus gemmatus, light photograph of opalescent scale patch, (d) P. gemmatus, habitus view.

Figure 8

Diffraction gratings. (a) Schematic of cuticular grating, (b) SEM of diffraction grating, Sphaeridiinae gen. sp. (Hydrophilidae), and (c) Sphaeridiinae gen. sp., habitus view with zero, first, second and third spectral orders labelled.

Figure 9

Modified diffraction gratings. (a) Pallodes sp. (Nitidulidae), SEM of bidirectional grating, (b) Pallodes sp. (Nitidulidae), ‘double’ spectral reflectance, (c) Aglyptinus tumerus (Leiodidae: Leiodinae), SEM of ‘interrupted’ grating, and (d) A. tumerus, habitus view.

Figure 10

Quasi-coherent scattering. (a) Heikertingerella sp. (Chrysomelidae: Alticinae), SEM of scattering structure, (b) Mychocerus sp. (Cerylonidae), SEM of scattering structure, and (c) Mychocerus sp. (Cerylonidae), habitus view showing diffuse reflectance.

Figure 11

Approximate phylogenetic distributions of iridescence mechanisms in Coleoptera. Triangles represent diffraction gratings; squares represent multilayer reflectors; circles represent three-dimensional photonic crystals. Tree simplified from that presented by Hunt et al. (2007).