Evolution of brilliant iridescent feather nanostructures.

The brilliant iridescent plumage of birds creates some of the most stunning color displays known in the natural world. Iridescent plumage colors are produced by nanostructures in feathers and have evolved in diverse birds. The building blocks of these structures-melanosomes (melanin-filled organelles)-come in a variety of forms, yet how these different forms contribute to color production across birds remains unclear. Here, we leverage evolutionary analyses, optical simulations, and reflectance spectrophotometry to uncover general principles that govern the production of brilliant iridescence. We find that a key feature that unites all melanosome forms in brilliant iridescent structures is thin melanin layers. Birds have achieved this in multiple ways: by decreasing the size of the melanosome directly, by hollowing out the interior, or by flattening the melanosome into a platelet. The evolution of thin melanin layers unlocks color-producing possibilities, more than doubling the range of colors that can be produced with a thick melanin layer and simultaneously increasing brightness. We discuss the implications of these findings for the evolution of iridescent structures in birds and propose two evolutionary paths to brilliant iridescence.

Iridescence and hydrophobicity have no clear delineation that explains flower petal micro-surface.

Plant organs including flowers and leaves typically have a variety of different micro-structures present on the epidermal surface. These structures can produce measurable optical effects with viewing angle including shifts in peak reflectance and intensity; however, these different structures can also modulate hydrophobic properties of the surfaces. For some species optical effects have been proposed to act as signals to enhance pollination interactions, whilst the ability to efficiently shed water provides physiological advantages to plants in terms of gas exchange and reducing infections. Currently, little is known about epidermal surface structure of flowering plants in the Southern Hemisphere, and how micro-surface may be related with either hydrophobicity or visual signalling. We measured four Australian native species and two naturalised species using a combination of techniques including SEM imaging, spectral sampling with a goniometer and contact angle measurements. Spectral data were evaluated in relation to published psychophysics results for important pollinators and reveal that potential visual changes, where present, were unlikely to be perceived by relevant pollinators. Nevertheless, hydrophobicity also did not simply explain petal surfaces as similar structures could in some cases result in very different levels of water repellency.

Comprehensive analysis of retroreflection in Papilio crino Fabricius, 1792 wings.

Multilayer thin-film structures in the wings of a butterfly; Papilio crino produce a colourful iridescence from reflected light. In this investigation, scanning electron microscope images show both the concave cover scales and pigmented air-chamber ground scales. The microstructures with the concavities retroreflect incident light, thus causing the double reflection. This gives rise to both the colour mixing and polarisation conversion clearly depicted in the optical images. The result of the numerical and theoretical analysis via the CIELAB, and optical reflection and transmission of light through the multilayer stacks with the use of transfer method show that the emerging colouration on the Papilio crino is structural and is due to the combination of colours caused by multiple bounces within the concavities. The butterfly wing structure can be used as the template for designing the photonic device.

Evolution of Insect Iridescence: Origins of Three-Dimensional Photonic Crystals in Weevils (Coleoptera: Curculionoidea).

A variety of photonic mechanisms give rise to iridescence and other structural colors in insects. In weevils (Coleoptera: Curculionoidea), iridescence is created by the most complex of these mechanisms, the three-dimensional photonic crystal. These self-assembling crystals take the form of triply periodic networks with single diamond or single gyroid symmetries and have been the subject of many descriptive studies based on individual species (often on a single specimen). To determine how these extraordinary nanostructures have evolved, we conduct the first comparative study of photonic crystals and setal nanostructure across Curculionoidea. By integrating structural data with newly available phylogenetic information, we demonstrate that-despite their widespread geographical and taxonomic distribution-three-dimensional photonic crystals appear to have evolved only once in weevils, in the common ancestor of a clade comprising the current subfamilies Entiminae and Cyclominae. Flattened, hollow setae with an unordered, spongy network in the lumen appear to be a necessary precursor to three-dimensional photonic crystals; we propose an evolutionary pathway by which this transformation has occurred.

Photonic crystal micro-pixelation and additive color mixing in weevil scales.

The origin of the brilliant near angle-independent coloration of the weevil Eupholus chevrolati was investigated by a combination of optical and electron microscopy tools, photonic band structure calculations, and color mixing analysis. Optical microscopy and scanning micro-spectroscopy revealed the presence of micrometer-sized red, yellow, green, and blue reflective pixels covering the entire exoskeleton of the weevil. Scanning electron microscopy in combination with focused ion beam milling showed that each micro-pixel consisted of a diamond-based photonic crystal structure and the different reflective colors were the result of different orientations of the photonic crystal. Color mixing analysis was used to study the collective behavior of the reflective micro-pixels. A pointillist, additive color-mixing scheme of the reflective photonic crystal micro-pixels was determined as the origin of the weevil's bright and near angle-independent yellow-green coloration.