Responses to continuous spectral scans in the ultraviolet (uv) have been measured intracellularly from the central retinula cells R7 and R8 in the fly (Musca, female). The spectral sensitivities thus obtained have a resolution limited by the bandwidth of the light supplied by the mono-chromator (0.3–1.5 nm).
One class of R7 cells, classified as 7y (Kirschfeld et al. 1978) shows three conspicuous peaks of sensitivity at 337, 355 and 373 nm (Fig. 3a). The underlying R8 cells (8y) also show three peaks but at slightly shorter wavelengths — 334, 350, and 369 nm (Fig. 3c), coinciding with those seen in the peripheral photoreceptors R1–6 (Gemperlein et al. 1980; Kirschfeld et al. 1982). Another class of R7 cells (7p) showed a spectral sensitivity function with a single peak at 330 nm. The underlying R8 cells (8p) also show a single-peaked function with maximum at 460 nm (Fig. 3b and d).
The results are interpreted as providing evidence for the hypothesis that uv sensitivity in 7y and 8y cells is conferred by a uv absorbing sensitising pigment similar to that demonstrated in R1-6 cells. The spectra of both 7p and 8p cells can be simply interpreted as deriving directly from the absorption of a rhodopsin with the appropriate λ max.
Musca-photoreceptors R7 and R8 uv sensitivity Sensitising pigment