Many Morpho butterflies are coloured in metallic, shimmering shades of blue and green. These colours are not due to pigmentation but due to iridescence: the extremely fine lamellated scales covering the Morpho's wings reflect incident light repeatedly at successive layers leading to interference effect that depends on both the wavelengths and angle of incidence. Thus the colours produced vary with viewing angle. However they are actually surprisingly uniform, perhaps due to the tetrahedral (diamondlike) structural arrangement of scales or diffraction from overlying cell layers. This structure may be called photonic crystal.