High refractive index of melanin in shiny occipital feathers of a bird of paradise

Male Lawes's Parotia, a bird of paradise, use the highly directional reflection of the structurally colored, brilliant-silvery occipital feathers in their courtship display. As in other birds, the structural coloration is produced by ordered melanin pigmentation. The barbules of the Parotia's occipital feathers, with thickness ~3 µm, contain 6–7 layers of densely packed melanin rodlets (diameter ~0.25 µm, length ~2 µm). The effectively ~0.2 µm thick melanin layers separated by ~0.2 µm thick keratin layers create a multilayer interference reflector. Reflectance measurements yielded peak wavelengths in the near-infrared at ~1.3 µm, i.e., far outside the visible wavelength range. With the Jamin-Lebedeff interference microscopy method recently developed for pigmented media, we here determined the refractive index of the intact barbules. We thus derived the wavelength dependence of the refractive index of the barbules' melanin to be 1.7–1.8 in the visible wavelength range. Implementing the anatomical and refractive index data in an optical multilayer model, we calculated the barbules' reflectance, transmittance and absorptance spectra, thereby confirming measured spectra

Spatially modulated structural colour in bird feathers.

Eurasian Jay (Garrulus glandarius) feathers display periodic variations in the reflected colour from white through light blue, dark blue and black. We find the structures responsible for the colour are continuous in their size and spatially controlled by the degree of spinodal phase separation in the corresponding region of the feather barb. Blue structures have a well-defined broadband ultra-violet (UV) to blue wavelength distribution; the corresponding nanostructure has characteristic spinodal morphology with a lengthscale of order 150 nm. White regions have a larger 200 nm nanostructure, consistent with a spinodal process that has coarsened further, yielding broader wavelength white reflectance. Our analysis shows that nanostructure in single bird feather barbs can be varied continuously by controlling the time the keratin network is allowed to phase separate before mobility in the system is arrested. Dynamic scaling analysis of the single barb scattering data implies that the phase separation arrest mechanism is rapid and also distinct from the spinodal phase separation mechanism i.e. it is not gelation or intermolecular re-association. Any growing lengthscale using this spinodal phase separation approach must first traverse the UV and blue wavelength regions, growing the structure by coarsening, resulting in a broad distribution of domain sizes

Spectral characteristics and regionalization of the eyes of Diptera, especially Tabanidae

The physical origin of the color patterns in the eyes of Tabanidae is well understood: a stack of layers in the facet lenses acts as an interference reflectance filter (Bernard & Miller 1968). This causes a reduced transmittance in specific wavelength bands. Because a facet lens focuses light into the underlying rhabdomeres, where the visual pigment molecules can absorb the incident light, the spectrally affected transmittance presumably causes a modified spectral sensitivity of the photoreceptors. We have investigated this conjecture by measuring reflectance spectra from the golden-greenish shining facets of the blinding breeze fly Chrysops relictus. By combining the associated transmittance spectrum with various visual pigment spectra, we conclude that noticeable shifts in photoreceptor sensitivity spectra can only occur with visual pigments that have absorption spectra peaking around 540 nm, i.e. in the green, in accordance with ERG measurements of the related horsefly Haematopota pluvialis (Kirschfeld 1986)

Quantifying the refractive index dispersion of a pigmented biological tissue using Jamin-Lebedeff interference microscopy

<p>Jamin-Lebedeff polarizing interference microscopy is a classical method for determining the refractive index and thickness of transparent tissues. Here, we extend the application of this method to pigmented, absorbing biological tissues, based on a theoretical derivation using Jones calculus. This novel method is applied to the wings of the American Rubyspot damselfly, Hetaerina americana. The membranes in the red-colored parts of the damselfly's wings, with a thickness of similar to 2.5 mu m, contain a pigment with maximal absorption at similar to 490 nm and a peak absorbance coefficient of similar to 0.7 mu m(-1). The high pigment density causes a considerable and anomalous dispersion of the refractive index. This result can be quantitatively understood from the pigment absorbance spectrum by applying the Kramers-Kronig dispersion relations. Measurements of the spectral dependence of the refractive index and the absorption are valuable for gaining quantitative insight into how the material properties of animal tissues influence coloration.</p>