Fixed structures A number of fixed structures can create structural colours, by mechanisms including diffraction gratings, selective mirrors, photonic crystals, crystal fibres and deformed matrices. When the bird moves the colour switches sharply between these two colours, rather than drifting iridescently. During courtship, the male bird systematically makes small movements to attract females, so the structures must have evolved through
sexual selection.
Photonic crystals can be formed in different ways. In
Parides sesostris, the emerald-patched cattleheart butterfly, photonic crystals are formed of arrays of nano-sized holes in the chitin of the wing scales. The holes have a diameter of about 150
nanometres and are about the same distance apart. The holes are arranged regularly in small patches; neighbouring patches contain arrays with differing orientations. The result is that these emerald-patched cattleheart scales reflect green light evenly at different angles instead of being iridescent. In
Lamprocyphus augustus, a weevil from
Brazil, the chitin exoskeleton is covered in iridescent green oval scales. These contain diamond-based crystal lattices oriented in all directions to give a brilliant green coloration that hardly varies with angle. The scales are effectively divided into
pixels about a micrometre wide. Each such pixel is a single crystal and reflects light in a direction different from its neighbours.
Selective mirrors to create interference effects are formed of micron-sized bowl-shaped pits lined with multiple layers of chitin in the wing scales of
Papilio palinurus, the
emerald swallowtail butterfly. These act as highly selective
mirrors for two wavelengths of light. Yellow light is reflected directly from the centres of the pits; blue light is reflected twice by the sides of the pits. The combination appears green, but can be seen as an array of yellow spots surrounded by blue circles under a microscope. The chitin walls of the hollow bristles form a hexagonal honeycomb-shaped photonic crystal; the hexagonal holes are 0.51 μm apart. The structure behaves optically as if it consisted of a stack of 88 diffraction gratings, making
Aphrodita one of the most iridescent of marine organisms. created by random nanochannels
Deformed matrices, consisting of randomly oriented nanochannels in a spongelike
keratin matrix, create the diffuse non-iridescent blue colour of
Ara ararauna, the
blue-and-yellow macaw. Since the reflections are not all arranged in the same direction, the colours, while still magnificent, do not vary much with angle, so they are not iridescent. '' berries
Spiral coils, formed of
helicoidally stacked
cellulose microfibrils, create
Bragg reflection in the "marble berries" of the African herb
Pollia condensata, resulting in the most intense blue coloration known in nature. The berry's surface has four layers of cells with thick walls, containing spirals of transparent cellulose spaced so as to allow
constructive interference with blue light. Below these cells is a layer two or three cells thick containing dark brown
tannins.
Pollia produces a stronger colour than the wings of
Morpho butterflies, and is one of the first instances of structural coloration known from any plant. Each cell has its own thickness of stacked fibres, making it reflect a different colour from its neighbours, and producing a
pixellated or
pointillist effect with different blues speckled with brilliant green, purple, and red dots. The fibres in any one cell are either left-handed or right-handed, so each cell
circularly polarizes the light it reflects in one direction or the other.
Pollia is the first organism known to show such random polarization of light, which, nevertheless does not have a visual function, as the seed-eating birds who visit this plant species are not able to perceive polarised light. Spiral microstructures are also found in
scarab beetles where they produce iridescent colours. petals exploit both yellow pigment and structural coloration.
Thin film with diffuse reflector, based on the top two layers of a buttercup's petals. The brilliant yellow gloss derives from a combination, rare among plants, of yellow pigment and structural coloration. The very smooth upper epidermis acts as a reflective and iridescent thin film; for example, in
Ranunculus acris, the layer is 2.7 micrometres thick. The unusual starch cells form a diffuse but strong reflector, enhancing the flower's brilliance. The curved petals form a paraboloidal dish which directs the sun's heat to the reproductive parts at the centre of the flower, keeping it some degrees Celsius above the ambient temperature.
Interference from multiple total internal reflections can occur in microscale structures, such as sessile water droplets and biphasic oil-in-water droplets as well as polymer microstructured surfaces. In this structural coloration mechanism, light rays that travel by different paths of
total internal reflection along an interface interfere to generate iridescent colour.
Variable structures '' Some animals including
cephalopods such as squid are able to vary their colours rapidly for both
camouflage and signalling. The mechanisms include reversible
proteins which can be switched between two configurations. The configuration of
reflectin proteins in
chromatophore cells in the skin of the
Doryteuthis pealeii squid is controlled by electric charge. When charge is absent, the proteins stack together tightly, forming a thin, more reflective layer; when charge is present, the molecules stack more loosely, forming a thicker layer. Since chromatophores contain multiple reflectin layers, the switch changes the layer spacing and hence the colour of light that is reflected. == Examples ==