Fig. 2.

Diagrams of an ommatidium of P. rapae crucivora. Light enters the facet lens and crystalline cone (fc) and then is focused into the rhabdom (rh). The rhabdom consists of rhabdomeres, distally of the photoreceptors R1–4, proximally of photoreceptors R5–8, and basally of photoreceptor R9. The rhabdom is partly surrounded by red pigment. Proximal to the rhabdom a tracheole exists, which acts as a tapetum. Left light propagates in the tapering rhabdom in waveguide modes; the profiles of the two lowest order modes are shown. Right simple compartment model of the type IIm ommatidium, of the male butterfly, with the R1–4 rhabdomeres taking up the distal 220 μm of the rhabdom, the R5–8 rhabdomeres the proximal 170 μm, and the R9 rhabdomere the basal 10 μm. The top distal 20 μm of the rhabdom contains a fluorescent, violet-absorbing pigment, and a deep-red pigment pigment is present from 60 to 280 μm. The lens shape in front of the rhabdom indicates the non-perfectly transparent dioptrical system