Circadian rhythm entrainment of the jewel wasp, Nasonia vitripennis, by antagonistic interactions of multiple spectral inputs.

Circadian light entrainment in some insects is regulated by blue-light-sensitive cryptochrome (CRY) protein that is expressed in the clock neurons, but this is not the case in hymenopterans. The hymenopteran clock does contain CRY, but it appears to be light-insensitive. Therefore, we investigated the role of retinal photoreceptors in the photic entrainment of the jewel wasp Nasonia vitripennis. Application of monochromatic light stimuli at different light intensities caused phase shifts in the wasp's circadian activity from which an action spectrum with three distinct peaks was derived. Electrophysiological recordings from the compound eyes and ocelli revealed the presence of three photoreceptor classes, with peak sensitivities at 340 nm (ultraviolet), 450 nm (blue) and 530 nm (green). An additional photoreceptor class in the ocelli with sensitivity maximum at 560-580 nm (red) was found. Whereas a simple sum of photoreceptor spectral sensitivities could not explain the action spectrum of the circadian phase shifts, modelling of the action spectrum indicates antagonistic interactions between pairs of spectral photoreceptors, residing in the compound eyes and the ocelli. Our findings imply that the photic entrainment mechanism in N. vitripennis encompasses the neural pathways for measuring the absolute luminance as well as the circuits mediating colour opponency.

Butterfly blues and greens caused by subtractive colour mixing of carotenoids and bile pigments.

Butterflies often have conspicuously patterned wings, due to pigmentary and/or structurally wing scales that cover the wing membrane. The wing membrane of several butterfly species is also pigmentary coloured, notably by the bile pigments pterobilin, pharcobilin and sarpedobilin. The absorption spectra of the bilins have bands in the ultraviolet and red wavelength range, resulting in blue-cyan colours. Here, a survey of papilionoid and nymphalid butterflies reveals that several species with wings containing bile pigments combine them with carotenoids and other short-wavelength absorbing pigments, e.g., papiliochrome II, ommochromes and flavonoids, which creates green-coloured patterns. Various uncharacterized, long-wavelength absorbing wing pigments were encountered, particularly in heliconiines. The wings thus exhibit quite variable reflectance spectra, extending the enormous pigmentary and structural colouration richness of butterflies.

Polarized iridescence of the tropical carpenter bee, Xylocopa latipes.

The tropical carpenter bee, Xylocopa latipes, has metallic-reflecting, iridescent wings. The wing reflectance spectra for TE- and TM-polarized light depend on the angle of light incidence in a way characteristic for dielectric multilayers. Anatomy indicates the presence of melanin multilayers in the wing's chitinous matrix. A simple optical model of melanin multilayers explains the angle dependence of the wing reflectance spectra. The wing reflections that occur upon oblique illumination exhibit colourful and strongly polarized light patterns, which may mediate intraspecific signaling and mutual recognition by conspecifics.

Subtractive colour mixing with bile pigments creates the rich wing palette of Graphium weiskei butterflies.

The wings of the purple spotted swallowtail Graphium weiskei are marked by an unusual bright colour pattern. Spectrophotometry on G. weiskei wings demonstrated the presence of a pigment with an absorption spectrum (peak wavelength λmax=676 nm) similar to that of the bile pigment sarpedobilin in the wings of the congeneric Graphium sarpedon (λmax=672 nm). Sarpedobilin alone causes cyan-blue wing areas, but the green-coloured areas of G. sarpedon wings result from subtractive colour mixing with the carotenoid lutein. Reflectance spectra of the blue-coloured areas of G. weiskei wings indicate that sarpedobilin is mixed with the short-wavelength-absorbing papiliochrome II. An enigmatic pigment, tentatively called weiskeipigment (λmax=580 nm), enhances the saturation of the blue colour. Weiskeipigment causes a purple colour in areas where the sarpedobilin concentration is low. The wings of the related papilionid Papilio phorcas contain the bile pigment pharcobilin (λmax=604 nm), as well as another sarpedobilin (λmax=663 nm). The cyan to greenish wings of P. phorcas are due to phorcabilin and sarpedobilin mixed with papiliochrome II. A survey of known subspecies of G. weiskei as well as of congeneric Graphium species of the 'weiskei' group shows various degrees of subtractive colour mixing of bilins and short-wavelength absorbers (carotenoids and/or papiliochromes) in their wings. This study illuminates the underestimated role of bile pigments in butterfly wing colouration.

Pigmentary colouration of hairy carpenter bees, genus Xylocopa.

Carpenter bees can display distinct colouration patterns due to structural coloured wings and/or coloured hairs on their bodies. Females of the sexually dichromatic Xylocopa caerulea are marked by strongly blue-pigmented hairs on the head, thorax and abdomen. The thorax of female X. confusa is covered by yellow-pigmented hairs. The diffuse pigmentary colouration of the blue and yellow hairs is effectively enhanced by strongly scattering granules. The absorption spectrum of the blue pigment of X. caerulea has a maximum at 605 nm and is probably a bilin (a bile pigment). The absorption spectrum of the yellow pigment of X. confusa has a maximum at 445 nm and may be a pterin. The thoracic hairs of female X. confusa contain also a minor amount of the bilin. The reflectance spectra of the pigmented hairs suggest that the pigments are tuned to the spectral sensitivity of the bees' photoreceptors and provide spectral contrast with a green background.

Evolution of Insect Color Vision: From Spectral Sensitivity to Visual Ecology.

Color vision is widespread among insects but varies among species, depending on the spectral sensitivities and interplay of the participating photoreceptors. The spectral sensitivity of a photoreceptor is principally determined by the absorption spectrum of the expressed visual pigment, but it can be modified by various optical and electrophysiological factors. For example, screening and filtering pigments, rhabdom waveguide properties, retinal structure, and neural processing all influence the perceived color signal. We review the diversity in compound eye structure, visual pigments, photoreceptor physiology, and visual ecology of insects. Based on an overview of the current information about the spectral sensitivities of insect photoreceptors, covering 221 species in 13 insect orders, we discuss the evolution of color vision and highlight present knowledge gaps and promising future research directions in the field.

The wing scales of the mother-of-pearl butterfly, Protogoniomorpha parhassus, are thin film reflectors causing strong iridescence and polarization.

The dorsal wings of the mother-of-pearl butterfly, Protogoniomorpha parhassus, display an angle-dependent pink, structural color. This effect is created by light interference in the lower lamina of the wing scales, which acts as an optical thin film. The scales feature extremely large windows that enhance the scale reflectance, because the upper lamina of ridges and cross-ribs is very sparse. Characteristic for thin film reflectors, the spectral shape of the reflected light strongly depends on the angle of light incidence, shifting from pink to yellow when changing the angles of illumination and observation from normal to skew, and also the degree of polarization strongly varies. The simultaneous spectral and polarization changes serve a possibly widespread, highly effective system among butterflies for intraspecific communication during flight.

Feather iridescence of Coeligena hummingbird species varies due to differently organized barbs and barbules.

Hummingbirds are perhaps the most exquisite bird species because of their prominent iridescence, created by stacks of melanosomes in the feather barbules. The feather colours crucially depend on the nanoscopic dimensions of the melanosome, and the displayed iridescence can distinctly vary, dependent on the spatial organization of the barbs and barbules. We have taken the genus Coeligena as a model group, with species having feathers that strongly vary in their spatial reflection properties. We studied the feather morphology and the optical characteristics. We found that the coloration of Coeligena hummingbirds depends on both the Venetian-blind-like arrangement of the barbules and the V-shaped, angular arrangement of the barbules at opposite sides of the barbs. Both the nanoscopic and microscopic organization of the hummingbird feather components determine the bird's macroscopic appearance.

The lesser purple emperor butterfly, Apatura ilia: from mimesis to biomimetics.

Until now, hues as dynamic as those adorning the Apatura emperor butterflies have never been encountered in the painting world. Unlike and unmatched by the chemical pigments traditionally found on the painter's palette, the emperor's wings are studded with strongly reflecting iridescent scales that are structured like those of the iconic Morpho butterflies. The scale ridges act as diffractive multilayers, giving rise to narrow-band reflectance spectra. All scales together create a vividly purple iridescent wing colouration that is observed within a narrow angular range only. Recently, synthetic structures analogous to the multilayer reflectors found on butterfly wings have been developed, referred to as effect pigments. Artists can obtain vital clues for how to adapt and adopt these challenging new materials for painting, by tracing the origin of biomimetics back to the ancient concept of mimesis and building on the knowledge accumulated by optical studies. By selecting various effect pigments, and using the lesser purple emperor butterfly, Apatura ilia, as exemplar, we have accurately mimicked the butterfly's iridescence in art. The resulting artwork, like the butterfly, fluctuates in perceived colour depending on the direction of illumination and viewing. These nature-inspired-colouration and biomimetic-application methods extend the canon of art.

Biophotonics of diversely coloured peacock tail feathers.

Peacock feathers feature a rich gamut of colours, created by a most sophisticated structural colouration mechanism. The feather barbules contain biophotonic structures consisting of two-dimensionally-ordered lattices of cylindrical melanosomes and air channels embedded in keratin. Here, we study the reflectance characteristics of the various peacock tail feather colours by applying bifurcated-probe- and micro-spectrophotometry and imaging scatterometry. We compare the experimental results with published anatomical SEM and TEM data, using a transfer-matrix based effective-medium multilayer model that includes the number and diameter of the melanosome rodlets and air channels, the lattice spacing and the keratin cortex thickness, together with the recently determined wavelength-dependence of the refractive indices of keratin and melanin. Slight variations in the parameter values cause substantial changes in the spectral position and shape of the reflectance bands. We find that the number of layers crucially determines the number of peaks in the reflectance spectra. For a small number of melanosome layers, the reflectance band shape is particularly sensitive to the properties of the uppermost layer, which provides a simple mechanism for tuning the feather colours.

The red admiral butterfly's living light sensors and signals.

We studied the wing colouration and the compound eyes of red admiral butterflies with optical methods. We measured reflectance spectra of the wing and scales of Vanessa atalanta and modelled the thin film reflectance of the wing membrane and blue scales. We utilized the eyeshine in the compound eye of Vanessa indica to determine the spectral and polarisation characteristics of its optical sensor units, the ommatidia. Pupil responses were measured with a large-aperture optophysiological setup as reduction in the eyeshine reflection caused by monochromatic stimuli. Processing of spectral and polarisation responses of individual ommatidia revealed a random array with three types of ommatidia: about 10% contain two blue-sensitive photoreceptors, 45% have two UV-sensitive photoreceptors, and 45% have a mixed UV-blue pair. All types contain six green receptors and a basal photoreceptor. Optical modelling of the rhabdom suggests that the basal photoreceptors have a red-shifted sensitivity, which might enhance the red admiral's ability to discriminate red colours on the wing. Under daylight conditions, the red shift of the basal photoreceptor is ∼30 nm, compared to the rhodopsin spectrum template peaking at 520 nm, while the shift of green photoreceptors is ∼15 nm.

Conical epidermal cells cause velvety colouration and enhanced patterning in Mandevilla flowers.

The majority of angiosperms have flowers with conical epidermal cells, which are assumed to have various functions, such as enhancing the visual signal to pollinators, but detailed optical studies on how conical epidermal cells determine the flower's visual appearance are scarce. Here we report that conical epidermal cells of Mandevilla sanderi flowers effectively reduce surface gloss and create a velvety appearance. Owing to the reduction in surface gloss, the flower further makes more efficient use of floral pigments and light scattering structures inside the flower. The interior backscattering yields a cosine angular dependence of reflected light, meaning that the flowers approximate near-perfect (Lambertian) diffusers, creating a visual signal that is visible across a wide angular space. Together with the large flowers and the tilted corolla tips, this generates a distinct visual pattern, which may enhance the visibility to pollinators.

Vividly coloured poppy flowers due to dense pigmentation and strong scattering in thin petals.

The flowers of poppies (Papaveraceae) exhibit bright colours, despite their thin and floppy petals. We investigated the optical properties of flowers of Papaver rhoeas, P. dubium, Meconopsis cambrica and Argemone polyanthemos using a combined approach of anatomy, spectrophotometry and optical modelling. The petals of Papaver flowers are composed of only three cell layers, an upper and lower epidermal layer, which are densely filled with pigment, and an unpigmented mesophyll layer. Dense pigmentation together with strong scattering structures, composed of serpentine cell walls and air cavities, cause the striking poppy colours. We discuss how various aspects of the optical signal contribute to the flower's visibility to pollinators.

Iridescent colouration of male Anna's hummingbird (Calypte anna) caused by multilayered barbules.

The male Anna's hummingbird features a brightly reddish-pink reflecting gorget, due to large stacks of melanosomes in the feather barbules, arranged in layers separated by keratin. Direct observations together with detailed scatterometry demonstrated that the barbules reflect incident light in an approximately specular manner. The structural colouration is iridescent, i.e. varies with a changing angle of light incidence. Spectrophotometrical measurements of the barbule reflectance and absorbance can be well interpreted with calculated spectra obtained with a transfer matrix method for optical multilayers, using anatomical data and measured refractive index spectra. The organization of the reflectors as a Venetian blind presumably functions to create a high spectral contrast of the male's plumage during courtship.

The giant butterfly-moth Paysandisia archon has spectrally rich apposition eyes with unique light-dependent photoreceptor dynamics.

The palm borer moth Paysandisia archon (Burmeister, 1880) (fam. Castniidae) is a large, diurnally active palm pest. Its compound eyes consist of ~ 20,000 ommatidia and have apposition optics with interommatidial angles below 1°. The ommatidia contain nine photoreceptor cells and appear structurally similar to those in nymphalid butterflies. Two morphological ommatidial types were identified. Using the butterfly numbering scheme, in type I ommatidia, the distal rhabdom consists exclusively of the rhabdomeres of photoreceptors R1-2; the medial rhabdom has contributions from R1-8. The rhabdom in type II ommatidia is distally split into two sub-rhabdoms, with contributions from photoreceptors R2, R3, R5, R6 and R1, R4, R7, R8, respectively; medially, only R3-8 and not R1-2 contribute to the fused rhabdom. In both types, the pigmented bilobed photoreceptors R9 contribute to the rhabdom basally. Their nuclei reside in one of the lobes. Upon light adaptation, in both ommatidial types, the rhabdoms secede from the crystalline cones and pigment granules invade the gap. Intracellular recordings identified four photoreceptor classes with peak sensitivities in the ultraviolet, blue, green and orange wavelength regions (at 360, 465, 550, 580 nm, respectively). We discuss the eye morphology and optics, the photoreceptor spectral sensitivities, and the adaptation to daytime activity from a phylogenetic perspective.

Magnificent magpie colours by feathers with layers of hollow melanosomes.

The blue secondary and purple-to-green tail feathers of magpies are structurally coloured owing to stacks of hollow, air-containing melanosomes embedded in the keratin matrix of the barbules. We investigated the spectral and spatial reflection characteristics of the feathers by applying (micro)spectrophotometry and imaging scatterometry. To interpret the spectral data, we performed optical modelling, applying the finite-difference time domain (FDTD) method as well as an effective media approach, treating the melanosome stacks as multi-layers with effective refractive indices dependent on the component media. The differently coloured magpie feathers are realised by adjusting the melanosome size, with the diameter of the melanosomes as well as their hollowness being the most sensitive parameters that influence the appearance of the feathers.

Longwing (Heliconius) butterflies combine a restricted set of pigmentary and structural coloration mechanisms.

BACKGROUND: Longwing butterflies, Heliconius sp., also called heliconians, are striking examples of diversity and mimicry in butterflies. Heliconians feature strongly colored patterns on their wings, arising from wing scales colored by pigments and/or nanostructures, which serve as an aposematic signal. RESULTS: Here, we investigate the coloration mechanisms among several species of Heliconius by applying scanning electron microscopy, (micro)spectrophotometry, and imaging scatterometry. We identify seven kinds of colored scales within Heliconius whose coloration is derived from pigments, nanostructures or both. In yellow-, orange- and red-colored wing patches, both cover and ground scales contain wavelength-selective absorbing pigments, 3-OH-kynurenine, xanthommatin and/or dihydroxanthommatin. In blue wing patches, the cover scales are blue either due to interference of light in the thin-film lower lamina (e.g., H. doris) or in the multilayered lamellae in the scale ridges (so-called ridge reflectors, e.g., H. sara and H. erato); the underlying ground scales are black. In the white wing patches, both cover and ground scales are blue due to their thin-film lower lamina, but because they are stacked upon each other and at the wing substrate, a faint bluish to white color results. Lastly, green wing patches (H. doris) have cover scales with blue-reflecting thin films and short-wavelength absorbing 3-OH-kynurenine, together causing a green color. CONCLUSIONS: The pigmentary and structural traits are discussed in relation to their phylogenetic distribution and the evolution of vision in this highly interesting clade of butterflies.

Sparkling feather reflections of a bird-of-paradise explained by finite-difference time-domain modeling.

Birds-of-paradise are nature's prime examples of the evolution of color by sexual selection. Their brilliant, structurally colored feathers play a principal role in mating displays. The structural coloration of both the occipital and breast feathers of the bird-of-paradise Lawes' parotia is produced by melanin rodlets arranged in layers, together acting as interference reflectors. Light reflection by the silvery colored occipital feathers is unidirectional as in a classical multilayer, but the reflection by the richly colored breast feathers is three-directional and extraordinarily complex. Here we show that the reflection properties of both feather types can be quantitatively explained by finite-difference time-domain modeling using realistic feather anatomies and experimentally determined refractive index dispersion values of keratin and melanin. The results elucidate the interplay between avian coloration and vision and indicate tuning of the mating displays to the spectral properties of the avian visual system.

How to colour a flower: on the optical principles of flower coloration.

The coloration of flowers is due to the wavelength-selective absorption by pigments of light backscattered by structures inside the petals. We investigated the optical properties of flowers using (micro)spectrophotometry and anatomical methods. To assess the contribution of different structures to the overall visual signal of flowers, we used an optical model, where a petal is considered as a stack of differently pigmented and structured layers and we interpreted the visual signals of the model petals with insect vision models. We show that the reflectance depends, in addition to the pigmentation, on the petal's thickness and the inhomogeneity of its interior. We find large between-species differences in floral pigments, pigment concentration and localization, as well as floral interior structure. The fractions of reflected and transmitted light are remarkably similar between the studied species, suggesting common selective pressures of pollinator visual systems. Our optical model highlights that pigment localization crucially determines the efficiency of pigmentary filtering and thereby the chromatic contrast and saturation of the visual signal. The strongest visual signal occurs with deposition of pigments only on the side of viewing. Our systematic approach and optical modelling open new perspectives on the virtues of flower colour.

Coloration of the Chilean Bellflower, Nolana paradoxa, interpreted with a scattering and absorbing layer stack model.

MAIN CONCLUSION: An absorbing-layer-stack model allows quantitative analysis of the light flux in flowers and the resulting reflectance spectra. It provides insight in how plants can optimize their flower coloration for attracting pollinators. The coloration of flowers is due to the combined effect of pigments and light-scattering structures. To interpret flower coloration, we applied an optical model that considers a flower as a stack of layers, where each layer can be treated with the Kubelka-Munk theory for diffusely scattering and absorbing media. We applied our model to the flowers of the Chilean Bellflower, Nolana paradoxa, which have distinctly different-colored adaxial and abaxial sides. We found that the flowers have a pigmented, strongly scattering upper layer, in combination with an unpigmented, moderately reflecting lower layer. The model allowed quantitative interpretation of the reflectance and transmittance spectra measured with an integrating sphere. The absorbance spectrum of the pigment measured with a microspectrophotometer confirmed the spectrum derived by modeling. We discuss how different pigment localizations yield different reflectance spectra. The absorbing layer stack model aids in understanding the various constraints and options for plants to tune their coloration.