Variation in the circularly polarized light reflection of Lomaptera (Scarabaeidae) beetles

Nature-Inspired Helicoidal Nanocellulose-Based Multi-Compartment Assemblies with Tunable Chiroptical Properties

Cellulose-based nanocomposites are highly appealing for the development of next-generation sustainable functional materials. Although many advances have been made in this direction, the true potential of fibrillar nanocomposites has yet to be realized because available fabrication approaches are inadequate for achieving precise structural control at the sub-micrometer scale. Here a spray-assisted alignment methodology of cellulose nanofibrils is combined with the layer-by-layer assembly into an additive manufacturing process in which the alignment direction of each cellulose layer is rationally selected to achieve thin films with a helicoidal arrangement of the nanofibrils. The helicoidal structure of the films is verified by measuring the circular dichroism (CD) of the samples. The sign and position of the structural CD peak show that the handedness and the pitch of the chiral structures can be easily tuned by deliberately selecting simple parameters, such as the number of consecutive cellulose layers sprayed in the same direction, and the angle of rotation between successive stacks of layers. To the authors' knowledge, this approach is unique as it offers the possibility to prepare complex nanocomposite architectures with various nanoscale-controlled sub-structures from different anisometric objects, which is enabling novel designs of composite films with damage-resistant and/or optical filtering functionalities.

Geographical features of Tulipa suaveolens Roth (Liliaceae, Magnoliophyta) distribution by flower color across its European range

The range of the polychromous Tulipa suaveolens Roth comprises almost the whole Ponto-Caspian Steppe, from the south-east of Ukraine to western Kazakhstan. High variation in flower color is a unique feature of this species, but features of its geographical distribution remain unclear. We studied T. suaveolens tepal color variation in 56 natural populations across the European range. Tepal colors were detected from digital images using the Lab color model with two chromatic components, a (red color intensity) and b (yellow color intensity). A conclusion was made that, throughout the European range, an obvious T. suaveolens flower color gradient is expressed in the direction from the south-west to the north-east, along which the red chromatic component intensity decreases. A similar gradient is observed when the height above mean sea level increases. The chromatic component a was shown to have a spatial autocorrelation and to depend on the long-term bioclimatic environmental parameters, temperature and precipitation. Thus, the identified geographical trends in the T. suaveolens flower color distribution across the studied part of its range are a consequence of natural selection caused by these two abiotic factors. 

Structural colours in the frond of Microsorum thailandicum

Blue and near-ultraviolet structural colours have often been reported in understorey plants living in deep shade. While this intense blue coloration is very catchy to the eye of a human observer, there are cases in which structural colours can be hidden either by the scattered light interacting with pigments or because they are found in unexpected positions in the plants. Here, we show that the fronds of Microsorum thailandicum produce structural coloration on both the adaxial and abaxial epidermal surface. While cellulose helicoidal structures are responsible for this coloration in both epidermal layers, the reflected colours are consistently different: an intense blue reflection is found in the adaxial epidermis while red-shifted and less intense colours are observed in the abaxial epidermis, possibly suggesting photo-adaptation of the plant to the light environment. By comparing the optical properties of the fern with its anatomy we computed the theoretical reflection accounting for the presence of disorder in the cellulose helicoidal architecture.

Catalogue of the types of the Scarabaeidae in the National Museum of Natural History of Luxembourg (Coleoptera)

The types of Scarabaeidae deposited in the collection of the National Museum of Natural History of Luxembourg are reported for the first time along with some historic and taxonomic remarks: Entypophanabiapicata Moser, 1913; Metabolusthibetanus Moser, 1914 (currently, Pseudosymmachia); Autosericaannamensis Moser, 1915 (currently, Maladera); Euphoresiaalboparsa Moser, 1913; Hybocamentaferranti Moser, 1917; Microsericaflaveola Moser, 1911; Triodontalujai Moser, 1917 (currently, Triodontella); Trochalusferranti Moser, 1917; Anomalacondophora Ohaus, 1913 (currently, Mimela); Amaurinaferranti Moser, 1911 (currently, Leucocelis); Amaurinavittipennis Moser, 1909; Cetonia (Eucetonia) kolbei Curti, 1914; Lomapteradichropusviridipes Moser, 1908; Cosmovalgusferranti Moser, 1912.

Pitch profile across the cuticle of the scarab beetle Cotinis mutabilis determined by analysis of Mueller matrix measurements

Helicoidal structures of lamellae of nanofibrils constitute the cuticle of some scarab beetles with iridescent metallic-like shine reflecting left-handed polarized light. The spectral and polarization properties of the reflected light depend on the pitch of the helicoidal structures, dispersion of effective refractive indices and thicknesses of layers in the cuticle. By modelling the outer exocuticle of the scarab beetle Cotinis mutabilis as a stack of continuously twisted biaxial slices of transparent materials, we extract optical and structural parameters by nonlinear regression analysis of variable-angle Mueller-matrix spectroscopic data. Inhomogeneities in the beetle cuticle produce depolarization with non-uniformity in cuticle thickness as the dominant effect. The pitch across the cuticle of C. mutabilis decreased with depth in a two-level profile from 380 to 335 nm and from 390 to 361 nm in greenish and reddish specimens, respectively, whereas in a yellowish specimen, the pitch decreased with depth in a three-level profile from 388 to 326 nm.

Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles

A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles (Chrysina chrysargyrea and Chrysina optima), measured in the wavelength range 300-1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman's formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz's equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.

Circularly polarized reflection from the scarab beetle Chalcothea smaragdina: light scattering by a dual photonic structure

Helicoidal architectures comprising various polysaccharides, such as chitin and cellulose, have been reported in biological systems. In some cases, these architectures exhibit stunning optical properties analogous to ordered cholesteric liquid crystal phases. In this work, we characterize the circularly polarized reflectance and optical scattering from the cuticle of the beetle Chalcothea smaragdina (Coleoptera: Scarabaeidae: Cetoniinae) using optical experiments, simulations and structural analysis. The selective reflection of left-handed circularly polarized light is attributed to a Bouligand-type helicoidal morphology within the beetle's exocuticle. Using electron microscopy to inform electromagnetic simulations of this anisotropic stratified medium, the inextricable connection between the colour appearance of C. smaragdina and the periodicity of its helicoidal rotation is shown. A close agreement between the model and the measured reflectance spectra is obtained. In addition, the elytral surface of C. smaragdina possesses a blazed diffraction grating-like surface structure, which affects the diffuse appearance of the beetle's reflected colour, and therefore potentially enhances crypsis among the dense foliage of its rainforest habitat.