Fly photoreceptors; III. Angular sensitivity as a function of wavelength and the limits of resolution

Abstract

The angular sensitivity of single retinula cells of the eye of Calliphora and Eristalis has been measured by standard methods over the range of wavelengths that are visible to these flies. The point of using different wavelengths is to test the dependence of the angular sensitivity upon the width of the Airy disk of light which is focused on the receptors. The width of the Airy disk is determined by diffraction and proportional to wavelength. The seven best Calliphora retinula cells had a mean acceptance angle of ∆ρv = 1.66 ± 0.22° s. d. in the vertical plane and ∆ρH = 1.44 ± 0.31° s. d. in the horizontal plane. The acceptance angle is independent of wavelength, and also approaches the theoretical lower limit inferred from the width of the Airy disk at 500 nm. This unexpected result is explained optically. In the dronefly Eristalis the retinula cells with a single spectral peak near 350 nm (and therefore inferred to be cell 7 or 8) have values of ∆ρH = 1.16 ± 0.23° s. d. and ∆ρv = 1.10 ± 0.17° s. d. at 350 nm and ∆ρH = 1.24 ± 0.31° s. d. and ∆ρv = 1.19 ± 0.26° s. d. at 450 nm. Retinula cells 1-6 of Eristalis, however, have a larger ∆ρ which is independent of wavelength. The difference in ∆ρ between the u. v. receptors and the cells 1-6 in Eristalis is explained by the smaller rhabdomeres of the former, because the two types of receptors share a common lens. In the most distal transverse sections, rhabdomeres 1-6 of Eristalis have major and minor diameters of 1.32 ± 0.11 μm s. d. and 1.09 ± 0.06 um s. d. (N = 12). Rhabdomere 7 has major and minor diameters of 0.74 ± 0.06 μm s. d. and 0.64 ± 0.08 μm s. d (N = 12). The observed values of ∆ρ for cells 1-6 are predicted from a simple theory based on the width of the Airy disk and the receptor size (Kuiper 1966) which predicts that ∆ρ is independent of wavelength. For cell 7 an additional factor is introduced whereby there is an effect of wavelength and the cross-section of the rhabdomere is effectively reduced at longer wavelengths.