Angular and spectral sensitivity of fly photoreceptors. II. Dependence on facet lens F-number and rhabdomere type in Drosophila.

A wave-optical model for the integrated facet lens-rhabdomere system of fly eyes is used to calculate the effective light power in the rhabdomeres when the eye is illuminated with a point light source or with an extended source. Two rhabdomere types are considered: the slender rhabdomeres of R7,8 photoreceptors and the wider, but tapering R1-6 rhabdomeres. The angular sensitivities of the two rhabdomere types have been calculated as a function of F-number and wavelength by fitting Gaussian functions to the effective light power. For a given F-number, the angular sensitivity broadens with wavelength for the slender rhabdomeres, but it stays approximately constant for the wider rhabdomeres. The integrated effective light power increases with the rhabdomere diameter, but it is for both rhabdomere types nearly independent of the light wavelength and F-number. The results are used to interpret the small F-number of Drosophila facet lenses. Presumably the small head puts a limit to the size of the facet lens and favors a short focal length.