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. 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. Surface gratings, consisting of ordered surface features due to exposure of ordered muscle cells on cuts of meat. The structural coloration on meat cuts appears only after the ordered pattern of muscle fibrils is exposed and light is diffracted by the proteins in the fibrils. The coloration or wavelength of the diffracted light depends on the angle of observation and can be enhanced by covering the meat with translucent foils. Roughening the surface or removing water content by drying causes the structure to collapse, thus, the structural coloration to disappear. 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