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Fig. 1. Diagram of an ommatidium and pattern of ultraviolet (UV), blue (B) and long wavelength (LW) opsin mRNA in the main retina of the monarch, Danaus plexippus (modified from Sauman et al., 2005). (A) Longitudinal view of an ommatidium. In nymphalids, the photoreceptor cell bodies that contribute to the fused rhabdom are organized into two tiers, composed of the R1–8 cells (tier I) and the R9 cell (tier II). Thick black lines, Ia. and Ib., indicate the approximate level from which the tangential sections of the R1–8 cells shown to the right were taken. c, cornea; cc, crystalline cone; n, photoreceptor cell nucleus; L, lamina; M, medulla. (B) Tangential views of ommatidial subtypes. Gray, purple and orange indicate the identity of the photoreceptor cells in which specific opsin mRNA expression is shown in the panels to the right. (C) Tangential section showing specific labeling of R1 and R2 cells with a digoxigenin-labeled antisense UV opsin riboprobe. Dashed circles indicate boundaries of identical individual ommatidia to those probed for B opsin mRNA in D. Scale bar, 25 µm. (D) Tangential section showing B opsin mRNA expression in an adjacent section to that shown in C. Dashed circles indicate identical ommatidia. Three subtypes of ommatidia are evident with respect to opsin expression in R1 and R2 photoreceptor cells: B–B, B–UV, UV–UV. Scale bar, 25 µm. (E) Tangential section showing LW opsin mRNA expression in all R3–8 cells (arrows). Scale bar, 25 µm.

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