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Abstract

The optical properties of the compound eye of Drosophila have been analysed using the optomotor reactions of flies with normal and mutant eye pigmentation. The stimulus was provided by cylindrical patterns with different periodic intensity distributions rotating at different speeds. The response consists of a torque about the vertical axis and was recorded under conditions of fixed flight. (Maximum reaction is about 0.04 dyn · cm). The transfer characteristics of the optical system are determined by the inter-ommatidial angle Δϕ, influencing the resolving power and by the width of the visual field of single ommatidia Δρ, influencing the response at high spatial frequencies. The values Δϕ = 4.6° and Δρ= 3.5° are obtained from stimulus-response experiments with Drosophila. They are independent of the presence of screening pigments. Differences in the response of flies with strong (+, se), weak (w a), and missing (w) pigmentation can be explained by the increased amount of scattered light in the pigment-deficient eyes. The overall intensities in the equally illuminated receptors are expected to be in the ratio 1∶1∶8∶25, respectively. The perception of motion depends only on the temporal, not on the spatial phase relations between periodic intensity variations in neighbouring ommatidia. Therefore the inhomogeneous distribution of the inter-ommatidial angle Δϕ changes the resolving power in different parts of the eye without changing the response to motion. Different simultaneous stimuli of equal strength in different parts of the eye are averaged in the perceptive system of Drosophila according to the number of ommatidia in these parts.