Spectral Sensitivities of Retinular Cells Measured in Intact, Living Flies by an Optical Method

The spectral sensitivity of the peripheral retinular cells R1-6 in nine species of intact flies was determined using non-invasive, optical measurements of the increase in reflectance that accompanies the pupillary response. Our technique is to chronically illuminate a localized region of the eye with a long wavelength beam, adjusted to bring pupillary scattering above threshold, then, after stabilization, to stimulate with monochromatic flashes. A criterion increase in scattering is achieved at each wavelength by adjusting flash intensity. Univariance of the pupillary response is demonstrated. Action spectra measured with this optical method are essentially the same as the published spectral sensitivity functions measured with intracellular electrophysiological methods for Calliphora, Drosophila, Eristalis, and Musca. This holds for both the long wavelength peak and the high sensitivity in the UV as was consistently found in all investigated fly species. Spectral sensitivity functions for R1-6 of hover flies (family Syrphidae) are quite different in different regions of the same eye. There can also be substantial differences between the two sexes of the same species. The ventral pole of the eye of female Allograpta contains receptors with a major peak at 450 nm, similar to those of Eristalis. However, the dorsal pole of the same eye contains receptors with a major peak at 495 nm, similar to those of Calliphora. Both dorsal and ventral regions of the male Toxomerus eye, and the ventral region of the female eye, contain only the 450 nm type of R1-6. However, the dorsal region of the female eye also contains another spectral type of receptor that is maximally sensitive at long wavelength. Eyes of both sexes of Allograpta contain a mixture of spectral types of receptors R1-6.