Fig. 1. (A) Diagram of an ommatidium of Papilio spp. (modified from Kitamoto et al., 1998). The ommatidium contains nine photoreceptor cells R1–R9. Photoreceptors R1–R4 are distal photoreceptors that contribute the rhabdomeral microvilli to the distal two-thirds of the rhabdom. Photoreceptors R5–R8 are proximal photoreceptors forming the rhabdom in the proximal one-third of the ommatidium. The basal photoreceptor, R9, contributes to the rhabdom at the region immediately distal to the basement membrane. UV, ultraviolet receptor; V, violet receptor: B, blue receptor; G, green receptors (two subtypes: SG, single-peaked green receptor; DG, double-peaked green receptor); R, red receptor; A, abnormally broadband receptor sensitive in the red and green ranges of the spectrum. (B) Relative sensitivities of five spectral types of photoreceptor.