FIGURE 3. Iridescence in the butterfly Morpho rhetenor.

a, Real colour image of the blue iridescence from a M. rhetenor wing. b, Transmission electron micrograph (TEM) images showing wing-scale cross-sections of M. rhetenor. c, TEM images of a wing-scale cross-section of the related species M. didius reveal its discretely configured multilayers. The high occupancy and high layer number of M. rhetenor in b creates an intense reflectivity that contrasts with the more diffusely coloured appearance of M. didius, in which an overlying second layer of scales effects strong diffraction⁴. Bars, a, 1 cm; b, 1.8 m; c, 1.3 m.