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Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin. However, the formation mechanism for the voided intra-scale structure is unknown. Here we report 3D x-ray nanotomography data for the voided chitin networks of intact white scales of Cyphochilus and Lepidiota stigma. Chitin-filling fractions are found to be 31 ± 2% for Cyphochilus and 34 ± 1% for Lepidiota stigma, indicating previous measurements overestimated their density. Optical simulations using finite-difference time domain for the chitin morphologies and simulated Cahn-Hilliard spinodal structures show excellent agreement. Reflectance curves spanning filling fraction of 5-95% for simulated spinodal structures, pinpoint optimal whiteness for 25% chitin filling. We make a simulacrum from a polymer undergoing a strong solvent quench, resulting in highly reflective (~94%) white films. In-situ X-ray scattering confirms the nanostructure is formed through spinodal decomposition phase separation. We conclude that the ultra-white beetle scale nanostructure is made via liquid–liquid phase separation. White beetle scales strongly scatter white light, whilst being very thin. Here, the authors measured the internal scale nanostructure for the beetles, Cyphochilus and L. stigma, and demonstrate that the optical structure can be simulated using liquid–liquid phase separation nanostructures, pointing to this as the formation mechanism.

Biological systems have a remarkable capability of synthesizing multifunctional materials that are adapted for specific physiological and ecological needs. When exploring structure-function relationships related to multifunctionality in nature, it can be a challenging task to address performance synergies, trade-offs, and the relative importance of different functions in biological materials, which, in turn, can hinder our ability to successfully develop their synthetic bioinspired counterparts. Here, we investigate such relationships between the mechanical and optical properties in a multifunctional biological material found in the highly protective yet conspicuously colored exoskeleton of the flower beetle, Torynorrhina flammea Combining experimental, computational, and theoretical approaches, we demonstrate that a micropillar-reinforced photonic multilayer in the beetle's exoskeleton simultaneously enhances mechanical robustness and optical appearance, giving rise to optical damage tolerance. Compared with plain multilayer structures, stiffer vertical micropillars increase stiffness and elastic recovery, restrain the formation of shear bands, and enhance delamination resistance. The micropillars also scatter the reflected light at larger polar angles, enhancing the first optical diffraction order, which makes the reflected color visible from a wider range of viewing angles. The synergistic effect of the improved angular reflectivity and damage localization capability contributes to the optical damage tolerance. Our systematic structural analysis of T. flammea's different color polymorphs and parametric optical and mechanical modeling further suggest that the beetle's microarchitecture is optimized toward maximizing the first-order optical diffraction rather than its mechanical stiffness. These findings shed light on material-level design strategies utilized in biological systems for achieving multifunctionality and could thus inform bioinspired material innovations.

Photonic structures in ordered, quasi-ordered or disordered forms have evolved across many different animal and plant systems. They can produce complex and often functional optical responses through coherent and incoherent scattering processes, often too, in combination with broadband or narrowband absorbing pigmentation. Interestingly, these systems appear highly tolerant of faults in their photonic structures, with imperfections in their structural order appearing not to impact, discernibly, the systems' optical signatures. The extent to which any such biological system deviates from presenting perfect structural order can dictate the optical properties of that system and, thereby, the optical properties that system delivers. However, the nature and extent of the optical costs and benefits of imperfect order in biological systems demands further elucidation. Here, we identify the extent to which biological photonic systems are tolerant of defects and imperfections. Certainly, it is clear that often significant inherent variations in the photonic structures of these systems, for instance a relatively broad distribution of lattice constants, can consistently produce what appear to be effective visual appearances and optical performances. In this article, we review previously investigated biological photonic systems that present ordered, quasi-ordered or disordered structures. We discuss the form and nature of the optical behaviour of these structures, focusing particularly on the associated optical costs and benefits surrounding the extent to which their structures deviate from what might be considered ideal systems. Then, through detailed analyses of some well-known 1D and 2D structurally coloured systems, we analyse one of the common manifestations of imperfect order, namely, the extent and nature of positional disorder in the systems' spatial distribution of layers and scattering centres. We use these findings to inform optical modelling that presents a quantitative and qualitative description of the optical costs and benefits of such positional disorder among ordered and quasi-ordered 1D and 2D photonic systems. As deviation from perfectly ordered structures invariably limits the performance of technology-oriented synthetic photonic processes, we suggest that the use of bio-inspired fault tolerance principles would add value to applied photonic technologies. This journal is

In the fabrication processes of photonic crystals, structural disorder arises in the synthesised structures from the limitations in the resolution of the synthesis processes. Elucidating the structural-disorder influence helps reducing its optical effects and allows using it in technological applications. Natural photonic structures are interesting examples to investigate due to their resilience to structural defects. The benefit and cost of these defects are so far not understood. In this contribution, we investigated the structural disorder in one-dimensional photonic crystals occurring on the wings of beetles. We performed optical simulations allowing to assess the optical benefit and cost of such disorder.

High resolution X-ray nano-tomography experiments are often limited to a few tens of micrometer size volumes due to detector size. It is possible, through the use of multiple overlapping tomography scans, to produce a large area scan which can encompass a sample in its entirety. Mounting and positioning regions to be scanned is highly challenging and normally requires focused ion beam approaches. In this work we have imaged intact beetle scale cells mounted on the tip of a needle using a micromanipulator stage. Here we show X-ray holotomography data for single ultra-white scales from the beetles Lepidiota stigma (L. stigma) and Cyphochilus which exhibit the most effective scattering of white light in the literature. The final thresholded matrices represent a scan area of 25 × 70 × 362.5 µm and 25 × 67.5 × 235µm while maintaining a pixel resolution of 25 nm. This tomographic approach allowed the internal structure of the scales to be captured completely intact and undistorted by the sectioning required for traditional microscopy techniques., Measurement(s)chitin • scaleTechnology Type(s)computed tomography • x-ray holotomographySample Characteristic - OrganismLepidiota • Cyphochilus Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12251450

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.

The scales covering the elytra and thorax of the male Hoplia coerulea scarab beetle are a specific case of natural photonic structure encased by a fluid-permeable envelope. These scales display several optical effects including structural colouration, fluid-induced colour changes, fluorescence emission and fluid-induced fluorescence changes. Although the photonic structure is not directly exposed to the environment, optical changes are known to be induced by liquids and vapours. These changes are controlled by the envelope that mediates exchanges with the surrounding environment. The optical system comprising the photonic structure and the envelope was previously termed “photonic cell”. Novel possibilities to develop functional photonic materials and smart coatings are offered by such a system through a bioinspired approach. This technological potential motivated us to analyse theoretically, through numerical simulations, the water-induced changes of optical properties of light emitters embedded in a photonic structure inspired by H. coerulea beetle’s scales.

Porous photonic structures found in several living organisms are known to display colour changes induced upon contact with liquids, vapours and gases. Usually these changes are due to physico-chemical phenomena such as the swelling of the structure enacted by the fluids, vapour physisorption on the pore walls, capillary condensation or a combination of them. Generally, the porous structures are open to outside, leading to fast fluid exchanges with the surrounding environment and consequently fast colour changes. In this article, we first introduce fluid-induced optical changes in living organisms exhibiting porous photonic structures. We explore then the potentiality of environmental ellipsometry for the first time in the context of natural photonic structures through the investigation of the optical response of the male cerulean chafer beetle Hoplia coerulea (Scarabaeidae) upon contact with water, 2-propanol and toluene vapours. In contrast with most of the investigated photonic structures, this beetle’s structure is encased by an envelope that mediates liquid exchanges with the environment. Such a study is of great interest in order to understand the underlying biological functions behind these changes as well as in order to develop bioinspired applications such as gas sensors and other environment-responsive coatings

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as non-linear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein these techniques are applied for the first time to the study of insects' natural fluorescence. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's colouration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A Third-Harmonic Generation signal is also detected, the intensity of which depends on the light polarisation state. The analysis of these non-linear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The anisotropy of the photonic structure, which causes additional tailoring of the beetle's optical responses, is confirmed by circularly polarised light and non-linear optical measurements., Comment: 12 pages, 4 figures, 1 table, 1 SI file

Photonic structures incorporated in lepidopteran scales are responsible for a very broad range of optical effects: iridescence, narrow-band reflection, large solid-angle scattering, polarisation effects, additive colour mixing and more. They have been the most investigated natural photonic structures for a long time. Such studies provide both understanding of the optical mechanisms and the biological functions behind these effects as well as inspiration for the design and development of novel photonic materials through a bioinspiration approach. In this chapter, research regarding structural colours in lepidopteran wing scales is reviewed through the classification of the related photonic structures. Selected examples of these structures are used to illustrate how such optical effects are brought about.

Layer-by-layer (LbL) assembly was used to fabricate a synthetic analogue of the color producing multilayered structure commonly found in many biological systems, particularly Coleoptera. The resulting iridescent films comprise multiple LbL-deposited layers designed to control color appearance through the materials' refractive indices and individual multilayer layer thicknesses. The fabricated systems, referred to as Bragg stacks, exhibited very similar optical behavior to the model and the beetle, selectively reflecting the desired color in a narrow band of visible wavelengths and displaying iridescent behavior.

Cellulose nanocrystals in water suspensions behave as lyotropic liquid crystals forming a chiral nematic phase above a critical concentration. Such an organization can be retained in solid films and give rise to an intense colored appearance. Here, we characterize their optical response by applying optical and scanning electron microscopy, imaging scatterometry, and angle-resolved reflectance measurements. We show that the experimental results are well explained by computational modeling using the finite-difference time-domain method, but slightly less well by Berremann’s analytical model.

The iridescent colors of Morpho butterflies have captured scientific intrigue for over a century. However, only recently photonic structures of the wing scales of Morpho butterflies have inspired new ideas in the diverse areas of technology including sensing. In this study, we performed theoretical and experimental evaluation of vapor-induced reflectance changes of the Morpho scales. These experiments provided additional details of the origin and the magnitude of vapor response selectivity in these natural photonic nanostructures and facilitated our design and fabrication of highly selective biomimetic photonic nanostructures.

The science of light and colour manipulation continues to generate interest across a range of disciplines, from mainstream biology, across multiple physics-based fields, to optical engineering. Furthermore, the study of light production and manipulation is of significant value to a variety of industrial processes and commercial products. Among the several key methods by which colour is produced in the biological world, this review sets out to describe, in some detail, the specifics of the method involving photonics in animal and plant systems; namely, the mechanism commonly referred to as structural colour generation. Not only has this theme been a very rapidly growing area of physics-based interest, but also it is increasingly clear that the biological world is filled with highly evolved structural designs by which light and colour strongly influence behaviours and ecological functions.

Interference phenomena are the source of some of the spectacular colors of animals and plants in nature. In some of these systems, the physical structure consists of an ordered array of layers with alternating high and low refractive indices. This periodicity leads to an optical band structure that is analogous to the electronic band structure encountered in semiconductor physics; namely, specific bands of wavelengths (the stop bands) are perfectly reflected. Here, we present a minimal model for optical band structure in a periodic multilayer and solve it using recursion relations. We present experimental data for various beetles, whose optical structure resembles the proposed model. The stop bands emerge in the limit of an infinite number of layers by finding the fixed point of the recursive relations. In order for these to converge, an infinitesimal amount of absorption needs to be present, reminiscent of the regularization procedures commonly used in physics calculations. Thus, using only the phenomenon of interference and the idea of recursion, we are able to elucidate the concepts of band structure and regularization in the context of experimentally observed phenomena, such as the high reflectance and the iridescent color appearance of structurally colored beetles.

H. coerulea’s elytra show a strong two-photon fluorescence signal, which depends on the presence of water in the photonic structure. A polarization-dependent Third Harmonic Generation signal was detected as well and thoroughly investigated.

The concentrically-layered photonic structure found in the tropical fruit Margaritaria nobilis serves as inspiration for photonic fibers with mechanically tunable band-gap. The fibers show the spectral filtering capabilities of a planar Bragg stack while the microscopic curvature decreases the strong directional chromaticity associated with flat multilayers. Elongation of the elastic fibers results in a shift of the reflection of over 200 nm.

The bright and intense blue-green coloration of the fruits of Margaritaria nobilis (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.

The precise control of light–matter interactions is crucial for the majority of known biological organisms in their struggle to survive. Many species have evolved unique methods to manipulate light in their environment using a variety of physical effects including pigment-induced, spectrally selective absorption or light interference in photonic structures that consist of micro- and nano-periodic material morphologies. In their optical performance, many of the known biological photonic systems are subject to selection criteria not unlike the requirements faced in the development of novel optical technology. For this reason, biological light manipulation strategies provide inspiration for the creation of tunable, stimuli-responsive, adaptive material platforms that will contribute to the development of multifunctional surfaces and innovative optical technology. Biomimetic and bio-inspired approaches for the manufacture of photonic systems rely on self-assembly and bottom-up growth techniques often combined with conventional top-down manufacturing. In this regard, we can benefit in several ways from highly sophisticated material solutions that have convergently evolved in various organisms. We explore design concepts found in biological photonic architectures, seek to understand the mechanisms underlying morphogenesis of bio-optical systems, aim to devise viable manufacturing strategies that can benefit from insight in biological formation processes and the use of established synthetic routines alike, and ultimately strive to realize new photonic materials with tailor-made optical properties. This talk is focused on the identification of biological role model photonic architectures, a brief discussion of recently developed bio-inspired photonic structures, including mechano-sensitive color-tunable photonic fibers and reconfigurable fluid micro-lenses. Potentially, early-stage results in studying and harnessing the structure-forming capabilities of living cells that lie at the origin of many species’ ability to grow photonic materials will also be presented.

The dorsal wing surfaces of papilionid butterflies of the nireus group are marked by bands of brilliant blue-green-colored cover scales. The thin, cuticular lower lamina of the scales acts as a blue reflector. The thick upper lamina forms a dense two-dimensional cuticular lattice of air cavities with a pigment acting as a long-pass optical filter. Reflectance spectra of small scale areas oscillate, but for large scale areas and the intact wing they are smooth. Theoretical modeling shows that the oscillations vanish for a scale ensemble with varying layer thicknesses and cavity dimensions. The scales combine in a subtle way structural and pigmentary coloration for an optical effect. (C) 2012 Optical Society of America

Iridescence is a property of structural color that is occasionally encountered in higher eukaryotes but that has been poorly documented in the prokaryotic kingdom. In the present work, we describe a marine bacterium, identified as Cellulophaga lytica , isolated from the surface of an anemone, that exhibits bright green iridescent colonies under direct epi-illumination. This phenomenon has not previously been investigated in detail. In this study, color changes of C. lytica colonies were observed at various angles of direct illumination or observation. Its iridescent green appearance was dominant on various growth media. Red and violet colors were also discerned on colony edges. Remarkable C. lytica bacterial iridescence was revealed and characterized using high-resolution optical spectrometry. In addition to this, by culturing other bacterial strains to which various forms of faintly iridescent traits have previously been attributed, we identify four principal appearance characteristics of structural color in prokaryotes. A new general classification of bacterial iridescence is therefore proposed in this study. Furthermore, a specific separate class is described for iridescent C. lytica strains because they exhibit what is so far a unique intense glitter-like iridescence in reflection. C. lytica is the first prokaryote discovered to produce the same sort of intense iridescence under direct illumination as that associated with higher eukaryotes, like some insects and birds. Due to the nature of bacterial biology, cultivation, and ubiquity, this discovery may be of significant interest for both ecological and nanoscience endeavors.

SUMMARY The wings of four papilionid butterfly species of the nireus group, Papilio bromius, P. epiphorbas, P. nireus and P. oribazus, are marked by blue-green coloured bands surrounded by black margins. The cover scales in the coloured bands contain a violet-absorbing, blue-fluorescing pigment. The fluorescence and absorbance spectra of the nireus group wings are very similar to those of the wings of the Japanese yellow swallowtail, Papilio xuthus, and thus the pigment is presumably papiliochrome II. The scale structures of P.xuthus are arranged irregularly, and both the fluorescence and light reflection are diffuse. In the nireus papilionids, the spatial fluorescence distribution of the scales is also diffuse, but the reflection is specular. The scales have a multilayered structure, consisting of two main laminae. We show that the papiliochrome II pigment in the upper lamina of the scales functions as a violet-blocking long-pass filter in front of the lower lamina, thus limiting the reflectance spectrum to the blue-green wavelength range. Optical modelling showed that the papiliochrome II filter effectively removes the angle dependency of the reflectance spectra – that is, it reduces the wing iridescence. The contribution of the fluorescence signal to the visual appearance is minor.

The surfaces of animals and plants are frequently adorned with a wealth of pattern, colour, and texture. Some insects for example, exhibit uniformly-coloured unpatterned white surfaces; some display bright iridescent blue or green hues; others may present varying shades of black, brown or selected combinations of spectral colours. Additionally, unseen by human observers, certain species' wing or body surfaces may produce strong ultra-violet or polarisation signatures. This review will introduce the characteristics of structural colour effects and describe examples of them in the natural world. It will summarise some of the recent work on a range of insect species, focussing predominantly on Lepidoptera as an exemplar order.

Iridescent color appearances are widespread in nature. They arise from the interaction of light with micron- and submicron-sized physical structures spatially arranged with periodic geometry and are usually associated with bright angle-dependent hues. Iridescence has been reported for many animals and marine organisms. However, iridescence has not been well studied in bacteria. Recently, we reported a brilliant “pointillistic” iridescence in colony biofilms of marine Flavobacteria that exhibit gliding motility. The mechanism of their iridescence is unknown. Here, using a multi-disciplinary approach, we show that the cause of iridescence is a unique periodicity of the cell population in the colony biofilm. Cells are arranged together to form hexagonal photonic crystals. Our model highlights a novel pattern of self-organization in a bacterial biofilm. ”Pointillistic” bacterial iridescence can be considered a new light-dependent phenomenon for the field of microbiology.

Combining vapour sensors into arrays is an accepted compromise to mitigate poor selectivity of conventional sensors. Here we show individual nanofabricated sensors that not only selectively detect separate vapours in pristine conditions but also quantify these vapours in mixtures, and when blended with a variable moisture background. Our sensor design is inspired by the iridescent nanostructure and gradient surface chemistry of Morpho butterflies and involves physical and chemical design criteria. The physical design involves optical interference and diffraction on the fabricated periodic nanostructures and uses optical loss in the nanostructure to enhance the spectral diversity of reflectance. The chemical design uses spatially controlled nanostructure functionalization. Thus, while quantitation of analytes in the presence of variable backgrounds is challenging for most sensor arrays, we achieve this goal using individual multivariable sensors. These colorimetric sensors can be tuned for numerous vapour sensing scenarios in confined areas or as individual nodes for distributed monitoring., Individual vapour sensors often suffer from poor selectivity, which hinders their broad applicability. Here, Potyrailo et al. fabricate individual sensors inspired by the Morpho butterfly capable of selectively detecting vapours in mixtures and with a variable moisture background.

Iridescent structural colors in biology exhibit sophisticated spatially-varying reflectance properties that depend on both the illumination and viewing angles. The classification of such spectral and spatial information in iridescent structurally colored surfaces is important to elucidate the functional role of irregularity and to improve understanding of color pattern formation at different length scales. In this study, we propose a non-invasive method for the spectral classification of spatial reflectance patterns at the micron scale based on the multispectral imaging technique and the principal component analysis similarity factor (PCASF). We demonstrate the effectiveness of this approach and its component methods by detailing its use in the study of the angle-dependent reflectance properties of Pavo cristatus (the common peacock) feathers, a species of peafowl very well known to exhibit bright and saturated iridescent colors. We show that multispectral reflectance imaging and PCASF approaches can be used as effective tools for spectral recognition of iridescent patterns in the visible spectrum and provide meaningful information for spectral classification of the irregularity of the microstructure in iridescent plumage.

A range of iridescent colour appearances are presented by male Swinhoe's pheasants' (Lophura swinhoii) mantle feathers. Two distinct regions of the open pennaceous portion of its feathers display particularly conspicuous angle-dependent reflection. A bright blue band appears in one region at normal incidence that spatially shifts to another at higher illumination angles. The two-dimensional photonic crystal-like nanostructures inside the barbules of these two regions are similar. However, this study found that the spatial variation in their colour appearance results from a continuously changing orientation of barbules with respect to the alignment of their associated barb. A multi-layered rigorous coupled-wave analysis approach was used to model the reflections from the identified intra-barbule structures. Well-matched simulated and measured reflectance spectra, at both normal and oblique incidence, support our elucidation of the origin of the bird's distinctive feather colour appearance.

The spectral and angle-dependent optical properties of two scarab beetle species belonging to the genus Chrysina are presented. The species display broadband reflectivity and selectively reflect left-circularly polarized light. We use electron microscopy to detail the left-handed, twisted lamellar structure present in these biological systems and imaging scatterometry to characterize their bidirectional reflectance distribution function. We show that the broadband nature of the beetles’ reflectance originates due to the range of pitch dimensions found in the structure.

Summary 1Structural coloration is a striking component of sexual ornamentation, and may function as a signal of mate quality. Although the proximate optical mechanisms are often well defined, we know much less about the morphological basis for intraspecific variation in structural colour. 2Males of the butterfly Colias eurytheme L. possess a thin-film interference array on their dorsal wing scales that generates a bright and iridescent ultraviolet (UV) signal. This signal is used in mate choice. 3Using scanning electron microscopy, we investigated the covariance between nano-structural architecture and UV reflectance in samples that were variously subject to either nutrient stress (using a larval host-plant manipulation), or thermal stress (using transient heat and cold shocks during the pupal period). We employed these two stressors to mimic natural stressful processes and to accentuate the variance in UV signal characteristics. 4Two primary structure–reflectance relationships were apparent. First, UV brightness increased with the density of scale ridges that bear the interference reflectors. Second, the breadth of above-wing angles for viewing the UV covaried with a measure of thin-film angular orientation. These relationships were, however, either limited to, or stronger among, males of the nutrient stress sample. 5Our results are consistent with a causal effect of developmental stress on nano-structural architecture and henceforth UV reflectance, but also suggest that the proximate basis for signal variation may be intimately linked to the nature of prevailing stressors.

This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) Knowledge Transfer Account programme grant EP/H50012X/1, and by QinetiQ Ltd.

Iridescence is an optical phenomenon whereby colour changes with the illumination and viewing angle. It can be produced by thin film interference or diffraction. Iridescent optical structures are fairly common in nature, but relatively little is known about their production or evolution. Here we describe the structures responsible for producing blue-green iridescent colour in Heliconius butterflies. Overall the wing scale structures of iridescent and non-iridescent Heliconius species are very similar, both having longitudinal ridges joined by cross-ribs. However, iridescent scales have ridges composed of layered lamellae, which act as multilayer reflectors. Differences in brightness between species can be explained by the extent of overlap of the lamellae and their curvature as well as the density of ridges on the scale. Heliconius are well known for their Müllerian mimicry. We find that iridescent structural colour is not closely matched between co-mimetic species. Differences appear less pronounced in models of Heliconius vision than models of avian vision, suggesting that they are not driven by selection to avoid heterospecific courtship by co-mimics. Ridge profiles appear to evolve relatively slowly, being similar between closely related taxa, while ridge density evolves faster and is similar between distantly related co-mimics.

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 beetleChalcothea 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 ofC. smaragdinaand 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 ofC. smaragdinapossesses 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.

The evolution of structural colour mechanisms in biological systems has given rise to many interesting optical effects in animals and plants. The instance of the scarab beetleChrysina resplendensis particularly distinctive. Its exoskeleton has a bright, golden appearance and reflects both right-handed and left-handed circularly polarized light concurrently. The chiral nanostructure responsible for these properties is a helicoid, in which birefringent dielectric planes are assembled with an incremental rotation. This study correlates details of the beetle's circularly polarized reflectance spectra directly with physical aspects of its structural morphology. Electron micrography is used to identify and measure the physical dimensions of the key constituent components. These include a chiral multilayer configuration comprising two chirped, left-handed helicoids that are separated by a birefringent retarder. A scattering matrix technique is used to simulate the system's optical behaviour in which the roles of each component of the morphological substructure are elucidated by calculation of the fields throughout its depth.

Evaporation induced self-assembled (EISA) thin films of cellulose nanocrystals (CNCs) have shown great potential for displaying structural colour across the visible spectrum. They are believed primarily to reflect left handed circularly polarised (LCP) light due to their natural tendency to form structures comprising left handed chirality. Accordingly the fabrication of homogenously coloured CNC thin films is challenging. Deposition of solid material towards the edge of a dried droplet, via the coffee-stain effect, is one such difficulty in achieving homogenous colour across CNC films. These effects are most easily observed in films prepared from droplets where observable reflection of visible light is localised around the edge of the dry film. We report here, the observation of both left and right hand circularly polarised (LCP/RCP) light in reflection from distinct separate regions of CNC EISA thin films and we elucidate how these reflections are dependent on the distribution of CNC material within the EISA thin film. Optical models of reflection are presented which are based on structures revealed using high resolution transmission electron microscopy (TEM) images of film cross sections. We have also employed spectroscopic characterisation techniques to evaluate the distribution of solid CNC material within a selection of CNC EISA thin films and we have correlated this distribution with polarised light spectra collected from each film. We conclude that film regions from which RCP light was reflected were associated with lower CNC concentrations and thicker film regions.

Scientific Reports 4, Article number: 6075 10.1038/srep06075 (2014); Published: August152014; Updated: December192014 This Article contains an error in the Acknowledgements section: “The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement n [291349] and USAF grant FA9550-10-1-0020.” should read: “The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n [291349] and USAF grant FA9550-10-1-0020 and the BBSRC David Phillips fellowship [BB/K014617/1].”

Brilliant iridescent colouring in male butterflies enables long–range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra–scale microstructures to overall butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles. Using a focused–laser technique, we investigated the absolute reflectivity and transmissivity associated with the single–scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide–angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100° in one plane and 15° in the other. We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected. Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single–scale microstructure to be n = (1.56+0.01) + (0.06 ±0.01)i. This figure is required for theoretical modelling of such microstructure systems.

What are Lepidoptera? Butterflies, moths and skippers form the order of insects known as Lepidoptera. Estimated to include more than 150,000 species, they are widely studied and have long been known to have highly complex morphologies, fascinating lifecycles and remarkable ecologies.Their broad wings are invariably covered with arrays of minute scales. Each scale is a thin, flattened, cuticular evagination from an individual cell in its wing's epithelium. Whether patterned or unadorned, and with few exceptions, it is these scales that are the pointillistic sources of all colour on lepidopteran wings. Their development offers a spectacularly diverse range of possible wing appearances, both in pattern and in colour.What is structural colour? The colour of each lepidopteran scale is created either by pigments, such as melanins, flavonoids, pterins, and so on, or by a periodic ultra-structure formed from the scale's cuticle. In some cases, there are both pigmentary and non-pigmentary contributions to an individual scale's appearance.The non-pigmentary source of colour, referred to as structural colour, is a very important component in the appearance of many different animal systems — examples are found in many other orders of insects, as well as in birds and aquatic animals. Some flora also make use of it. In Lepidoptera, the ecological selection pressures and the processes responsible for cuticle formation have led to a remarkable diversity of scale designs for producing structural colour.Structural colour utilizes the wave-nature of light. As a wave, light can experience wave superposition; that is, groups of waves may add together to reinforce or diminish their combined effect. For this to happen effectively and therefore to produce a distinct colour effect, there must be a definite structural order in the system; importantly, the physical dimension of this order, the period, must be on a par with the wavelength of light. This phenomenon is often referred to as interference and is identical to the mechanism that produces the iridescent colours in soap bubbles; other names for it include Bragg diffraction or coherent scattering.Why is it advantageous to be structurally coloured? Structural colour offers several key functionalities that are not accessible using pigmentation. The first, in broad terms, is that it provides the facility for a system, such as a butterfly wing, to control explicitly the way that light reflects from it, propagates within it or is transmitted through it.Clearly, as these aspects control visual appearance, from camouflaging to ultra-long distance signalling, and even to thermoregulation, it is a vitally important feature. As such, it has been a key variable in the evolution of very many different species. Such control over light extends not only to pretty much all aspects of the narrowness or breadth of its spectral purity, but also to the intensity of the colour, its perceived angle-dependence and also the angle over which it is visible. Strong linear or circular polarisation may also be incorporated into the colour signal if the appropriate scale structures are present.How are these sorts of morphological structures formed? The processes that determine the characteristic morphology of each butterfly or moth's colour pattern begin long before metamorphosis, while the creature is still a larva and its wings still undifferentiated internal imaginal disks. As the wing develops, its outer surface forms slender projections from widely spaced scale cells. The projections flatten, elongate and then develop series of periodic or aperiodic ultra-structures. These ultimately underpin the scale's function and appearance. A growing scale cell lays down a thin malleable outer layer of cuticle, firstly around itself (as the secretion and assembly of cuticle takes place extracellularly), then on the structures forming the inner layers of cuticle. When the cell finally dies back into the epithelium of the wing, the newly formed scale dries and hardens to leave the finished functional product.With some scale types, namely those that feature a form of two-dimensional periodicity within the specialised ridging on the scale's dorsal surface, drying-related elastic buckling is proposed as the mechanism by which the requisite periodic precision is attained. In other forms of scale, those which comprise a fully three-dimensional photonic crystal, the scale cell forms a three-dimensional network of smooth endoplasmic reticulum which it appears to use to control the three-dimensional structure and spacing of secreted cuticle.What are some good examples of structural colour in Lepidoptera? Let us start with Morpho butterflies since these are among the most well known and conspicuously coloured species. The intensely bright iridescent blue wings of some species of the Morpho family facilitate very long-range signalling (they can be seen from a quarter of a mile away). A highly periodic, one-dimensional multilayer, with alternating cuticle and air layers, is arranged in the discrete arrays formed by each scale's specialised ridging. These create a reflection of up to 80% of incident blue light and offer the wing an extremely broad angular visibility. Other Morpho species use a superficial layer of scales, on top of the blue reflecting ones, to diffract the reflected blue light out into even broader angles, increasing angular visibility still further.A similar sort of structure, with modified multilayer dimensions, is used to create a highly reflective signal from the wings of some ultra-violet iridescent Colias butterflies, such as C. eurytheme males. To our eyes, however, as we are unable to discern these shorter wavelengths, their wings appear yellow, because of the presence of pterin pigment granules packed between the structurally coloured ridges.Several of the gloss swallowtail butterflies, Papilio palinurus is one example (Figure 1Figure 1), exhibit wonderfully soft green-coloured banding across their wings. The green is entirely structural in origin. High magnification inspection, however, reveals curved multilayers beneath the surface of hundreds of 5 micron diameter concavities on each of the wings' green scales, which simultaneously reflect yellow from their centres and blue annuli from around their sides. Together they form juxtaposed pointillistic colour centres which, to our eyes, create the stimulus of the colour green (the same principle is used in many neo-impressionist paintings).Figure 1The Papilio palinurus butterfly, pictured here, simultaneously produces two structural colours (yellow and blue) from a single form of wing-scale ultra-structure. These pointillistic colour centres combine to produce the stimulus of green to our eyes.View Large Image | View Hi-Res Image | Download PowerPoint SlideOne further special feature of this structurally coloured system, and possibly the reason it evolved, is that only the blue colour component is significantly linearly polarised; linear polarisation is sometimes used for intra-specific signalling in Lepidoptera, so this system seems ideally designed to be cryptically green to, say, predator visual systems that do not see differences in the polarisation of light, while broadcasting linearly polarised blue signals to conspecifics.A separate small family of swallowtails, the Nireus group that is indigenous to the Afrotropics, concurrently employs a fluorescent pigment and not one, but two forms of specialised photonic structure: a multilayer and a two-dimensional photonic crystal slab. These carefully control the emission direction of the fluorescent light and make the butterfly a much brighter object than it would be without them. This is the first known example in which fluorescent pigment and colour-producing or colour-controlling nanostructures are so closely tied together.There are a vast number of other examples. Lepidopteran structural colour systems, and those in other orders too, truly exhibit a remarkable ability to control the flow of light and colour in so many ways and for so many purposes.Why is this subject worth investigating? Textbooks tell us that a colour pattern is a slice in time; a frozen moment in the dynamic processes of development and evolution. Genetic information is translated into the manipulation of cell systems and cuticle assembly to give final patterns and appearances.To understand these processes in Lepidoptera, we examine the biological components that underpin their wing colours and patterns. We seek to know how changes in genes and development can control colour and pattern; how larval food plant quality affects male sexual quality indicators; how the forms and relative weighting of inter-specific and intra-specific selection pressures affect wing colour intensity, hue, polarisation and angular visibility.A study of the biology of the wing colours and patterns of Lepidoptera is a study of evolution itself. Now, within this field, it is increasingly clear that structural colour, and the many ways in which it is produced, is a fundamental component.From the perspective of technological photonics, these lepidopteran systems are amazingly elaborate. They feature self-assembled highly complex biological engineering for photonic purposes (amongst others) that, given the limited range of constituent materials, exhibit an ingenuity of design that easily surpasses all but a few of our best technological efforts. Technology is keenly interested in the ideas and design principles that such natural photonic systems, such as those in certain Lepidoptera, are able to offer.Who knows how much photonics research time may be saved, or how many technological design solutions may be realised, simply by looking to nature's wing for inspiration?

For almost a century, the iridescence of tropical Morpho butterfly scales has been known to originate from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps. Here we describe a biological pattern of surface functionality that we have found in these photonic structures. This pattern is a gradient of surface polarity of the ridge structures that runs from their polar tops to their less-polar bottoms. This finding shows a biological pattern design that could stimulate numerous technological applications ranging from photonic security tags to self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As an important first step, this biomaterial property and our knowledge of its basis has allowed us to unveil a general mechanism of selective vapor response observed in the photonic Morpho nanostructures. This mechanism of selective vapor response brings a multivariable perspective for sensing, where selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors.

The departure from strict periodic order in two-phase dielectric materials can offer properties that are otherwise inaccessible to perfectly ordered photonic crystals. Herewith, we investigate the circular dichroism of the single gyroid photonic crystal in the presence of spatial distortions. FDTD simulations and microwave transmission measurements on 3D-printed replicas show that certain harmonic long-wavelength spatial distortions ("sinusoidal chirp") nearly doubles the imbalance of the circular polarisation reflectances, as well as significantly strengthens polarisation-incoherent reflectance. The observed changes are partially rationalised by comparison with simpler distortion models (linear chirp and tetragonal deformation) of the Gyroid.

Optimised multilayer structures and efficient coating designs have been found in several species of damselfly. We discuss these structures and the associated mechanisms responsible for the bright optical effects which they exhibit.

Visual appearance generates stimuli associated with many biological functions, including interspecies and intra species communication. A range of biological structural colour mechanisms has been identified. These mechanisms include highly periodic microstructures associated with bright and saturated colours, and amorphous structures which produce broadband colours and generally diffuse reflectances. In this chapter several highly functional amorphous structures found in biological systems are detailed, and their optical characteristics are described.

Here we discuss the vapour response observed in the iridescent scales of the Morpho sulkowskyi butterfly and the opportunities that may be achieved in a single nano-structured sensing unit rather than traditional sensor arrays.

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.