Differential opsin expression in Drosophila. Schematic drawings of photoreceptor cell arrangements in the three types of ommatidia that exist in the retina of Drosophila melanogaster, and differ in the choice of differentially expressed opsin paralogs. (a) Cross-section perspective. It should be noted that the R7 and R8 rhabdomeres are represented in the same plane for ease in representation. (b) Sagital section perspective. In pale-type ommatidia, the peripheral photoreceptors R1–R6 express the LW-opsin Rh1, the R7 photoreceptor cell expresses the UV-opsin paralog Rh3, and the R8 photoreceptor cell expresses the B-opsin paralog Rh5. Yellow differ from pale ommatidia by expression of the UV-opsin paralog Rh4 in the R7 cells, and LW-opsin paralog Rh6 in R8. Dorsal rim area (DRA) ommatidia express Rh3 in both the R7 and R8 cell .