Quantification of rhabdomeral Ops1-2-immunoreactivity (Ops1-2-ir) and Ops5-ir in lateral eye (LE) photoreceptors fixed under natural illumination during the night (N), between 18 and 20 h after sunrise, at sunrise (SR), and during the day (D), between 9 and 10 h after sunrise. The mean intensity of rhabdomeral Ops-ir in an ommatidium was determined by measuring the total intensity within ROI1 minus ROI2 divided by the area of ROI1 minus ROI2 (see insert). The mean intensity for each eye was determined by averaging data from at least eight separate ommatidia. Data are pooled from three separate experiments done during April, July and August in which LEs from 2–4 animals were analyzed in each experiment. Data are expressed as the mean intensity of rhabdomeral Ops-ir × 10–3 ± s.e.m. for the number of animals indicated in the columns. The significance of differences among time points was tested using a one-way ANOVA followed by a t-test. Significant differences are indicated with parentheses and an asterisk (P<0.001). The mean intensity of rhabdomeral Ops1-2-ir during the day is about 50% of that observed during the night and at sunrise. The mean intensity of rhabdomeral Ops5-ir does not change significantly from day to night.