Figure 3. A, cuticular lens–tapetum model: bold dots in the lens represent the centre of the arc described by the outer and inner surface of the lens; r1, the radius of curvature of the outer lens surface; r2, the radius of curvature of the inner surface; e1, the thickness of the lens; e2, the distance between the lens and the tapetum; d, the diameter of a spherical mirror. B, an example of the computer simulation of ray tracing: Lt, the sum of the total lengths of segments of rays passing through the rhabdom (shown by the grey segments); n, the total number of rays of incident light.