A diffraction theory of insect vision. I. An experimental investigation of visual acuity and image formation in the compound eyes of three species of insects

Abstract

Spike responses were recorded in the optic lobes and nerve cord of the locust and two species of Diptera in response to visual stimulation by moving patterns of black and white stripes. The minimum angular separation of adjacent stripes needed to evoke a threshold response was 0·3° in all three species. This value of resolution is higher by a factor of three to four times than the values obtained in behavioural experiments, the results of which have tended to show an agreement between minimum angle of resolution and angular inclination of adjacent ommatidia. Examination of eye slices by optical methods revealed a succession of images formed in depth in the three species studied. Of these the most superficial was a simple geometrical image, followed by deeper images formed by diffraction. The resolution afforded in the first image was of the order of 1°, which agrees with the value calculated from the telescope formula. The resolution was higher in the diffraction images, increasing with order. Two such images are formed within the photoreceptor zone of the eyes of Locusta and Calliphora . The highest-order image in each case afforded a resolution similar to that observed in the experiments with moving striped patterns performed on the living eye. The diffraction images are formed by the co-operation of many ommatidia, acting as multiple coherent sources, and have special properties, which are described. A particular property is that of sensitivity to orientation of striped patterns, giving preference to objects of radial symmetry. The relation of this property to earlier results of behavioural experiments is discussed. The physical derivation of these images is described in part II of this paper, by G. L. Rogers (1962), in which experiments on models simulating the eye structure are described.