The Ruteline genera Chrysina and Anoplognathus consists of brightly colored, often metallic iridescent species. The genus Chrysina ranges from the subtropical south of the United States to Ecuador and the genus Anoplognathus is endemic to Australia and Papua New Guinea. The adults of both genera are between 15-35mm in length (Anoplognathus viridiaeneus and Chrysina spectabilis are the largest species).
Adults of Chrysina feed on foliage in pine, juniper, or pine-oak forests between 1000-3000m elevation and their larvae live in rotting logs. They are most diverse in countries such as Honduras, where as many as 15 species can be found in a single location. Species of Anoplognathus are also known as Christmas beetles because adults emerge around Christmas time and could be found around lights in urban areas. Adults are defoliators of eucalypts and larva feed on organic matter in the soil.
Metallic iridescent species of these genera shine not because of chemical pigmentation or the incorporation of actual metals. Srinivasarao et al. (2011) found that the beetles use a unique helical structure that reflects light with a wavelength of 530 nanometers mixed with yellow light in a wavelength of 580 nanometers – and of only one polarization: left circular polarization.
The reflecting structure is produced from a cholesteric liquid crystal and consist predominately of three different polygonal shapes (hexagonal cells, pentagons and heptagons) whose percentages vary with the curvature of the exoskeleton. Due to the helical pitch of the cholesteric liquid crystal, the reflection is very metallic. The silver beetle reflects light across the entire visible spectrum whereas the golden beetle reflects light of wavelengths larger than 515 nanometers—similar to the reflection spectra for the actual metals.
Reflectance of circularly polarized light by beetle cuticle was first reported in beetles by Michelson (1911) in Chrysina resplendens; this phenomenon appears to be restricted to Scarabaeidae, occurring predominantly in Rutelinae, Scarabaeinae and Cetoniinae (Seago et al. 2009). Ischiosopha bifasciata (Cetoniinae) is a narrowband left-circular polarizer; Chrysophora chrysochlora (Rutelinae), a broadband left-circular polarizer; and Chrysina woodi (Rutelinae), an elliptical polarizer (Lowrey et al. 2007). Crowson (1981) and Thomas et al. (2007) suggests that metallic iridescent beetles are imperceptible because they reflect the coloration of their environment. The optical properties may confuse predators, causing them to misjudge the location of the insects.
In contrast to circular polarization in scarab beetles, the metallic iridescence in Buprestidae (also known as Jewel Beetles) is caused by only linear polarization by the comparable flat surfaces. The epicuticle consists of a stack of several layers and could be treated as multilayer interference reflector that creates a strong polarized iridescence (Stavenga et al. 2011).
During the formation of the epicuticle, parallel chitin layers with different refractive indexes are secreted by the epidermis and hardened during sclerotization. If the spacing of these layers approaches one-quarter the wavelength of visible light (approx. 380–750 nanometers), one or more colours will be produced by constructive interference (Land 1972).
Stavenga et al. (2011) suggest that in Chrysochroa fulgidissima the polarized iridescence exhibited by the reflecting elytra serves for intra-specific signaling between female and male. That spectral reflectance values of the jewel beetles are not very different from those of green leaves, rather suggests that the iridescence functions as anti-predator mechanism.
In the Curculionidae, three-dimensional crystalline structures producing gem-like reflectance were described by Parker et al. (2003), Welch et al. (2007) and Galusha et al. (2008). Most members of the Curculionidae, Brentidae and Attelabidae have pigmented or structurally coloured scales. In the conspicuously colored Curculionidae genera Lamprocyphus and Eupholus, the body surface is covered with blue, green or yellowish scales. In L. augustus each scale consist of a three-dimensional photonic structure. The arrangment of the structure resembles the diamond structure in carbon atoms (Bartl). Within a scale, the diamond lattice is positioned at different orientations, giving the beetle its green sheen from almost any angle.
Pouya et al. (2011) found that the scales in yellow elytral bands in Eupholus comprise highly ordered three-dimensional photonic crystal structures, whereas the scales of blue bands comprise less ordered structures. The differences in the contrasting structures of both systems are responsible for approximately angle-independent colour appearances in the far-field. The authors mentioned that co-existence of these two contrasting structures in the same single species is uncommon in natural biological systems. The coloration in curculionids may a warning to predators, it was suggested that the beetles are not palatable because they may accumulate plant toxins.
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