ABSTRACTThe uniform green colour of the under surface of the wing ofCallophrys rubiis due to the integrated effect of the iridescence from a mosaic of polygonal grains, 5.4μm. diameter and 1μm. thick. The ultrastructure of each grain is a simple cubic network, lattice parameter 0257μm. The iridescent colour results from this structure acting as a volume diffraction grating giving a peak reflectance at about 0.548μm., green, which is found to be close to the observed reflectance with a peak at 0.550μm. measured by a spectrom