Fig. 2. Ommatidia, filtering pigment distribution and pattern of opsin gene expression in lycaenid butterflies. (A) Longitudinal (left) and tangential views (right) of the two types of ommatidia in the ventral eye of P. icarus, non-pigmented (I) and red-pigmented (II). Purple pupillary pigments are also present distally all in R1–8 photoreceptor cells regulating the amount of light entering each ommatidium. c, cornea; cc, crystalline cone; 9, the ninth photoreceptor; tp, tapetum; L, lamina; M, medulla. (B) Red-filtering pigment in the lateral eye of P. icarus is present in some ommatidia (a) and absent in others (b). (C) Cartoon of the six ommatidial subtypes found in the ventral retina of L. rubidus with respect to the non-overlapping expression of UVRh (UV, gray), BRh1 (B1, dark blue) and BRh2 (B2, light blue) mRNAs in R1 and R2 photoreceptor cells. The LWRh mRNA (orange) is expressed in the R3–8 cells. Note: the red-filtering pigment (red dots) of L rubidus is coordinately expressed in the same ommatidia as those expressing BRh2. Experimental data upon which the cartoon is based are from Sison-Mangus et al. (Sison-Mangus et al., 2006).