Fig. 6. Optical path length difference and coherence length of the diurnal moth P. tristifica and the nocturnal moth E. kühniella. The numbers adjacent to the large symbols indicate facets in a meridional row (Fig. 5). The optical path length difference between rays passing through the facets marked by the number 2 and a central ray passing through facet 1, is smaller than the coherence length of light for the green-sensitive photoreceptors (l_c=2.8 µm). This also holds between rays passing through facet pairs 7 and 8, 13 and 14, and 16 and 17.