Visual pigment spectra from sensitivity measurements after chromatic adaptation of single dronefly retinula cells

Abstract

The sensitivity of single retinula cells 1–6 of the flyEristalis towards monochromatic light was measured in two types of adaptation experiments. First, the sensitivity was measured at various wavelengths after adaptation of the visual pigment to the photochemical equilibrium at a fixed wavelength (Figs. 3, 4, 5 and 6). Then, on the same cell, the sensitivity was measured by a test light of a fixed wavelength after adaptation at each wavelength, from 333 nm to 594 nm (Figs. 8 and 9). For this fixed test wavelength the isosbestic wavelength was selected. The relative absorption coefficients of the visual pigment rhodopsin (R) and its photoproduct metarhodopsin (M) were calculated. The absorption spectra are comparable to those found by photometric measurements on cells 1–6 of the same population ofEristalis (Stavenga, 1976). The calculations also show that the percentage absorption at peak wavelength (near 450 nm) is 85–95% for unpolarized light, and when the isosbestic point is taken to be 485 nm, the relative quantum efficiency for the M toR conversion is about 0.8 of that of theR toM conversion. Moreover, an absorption spectrum for a single pigment, with a lowβ peak near 360 nm, is found, irrespective of whether the retinula cell has a greater sensitivity in the UV than at the standard peak of longer wavelength. The additional sensitivity in the UV, which is characteristic of some retinula cells 1–6, is therefore apparently unrelated to the basic properties of the visual pigment.