A theory of movement perception has been proposed to explain the optomotor responses of the housefly (
Musca domestica
), and this has been tested by McCann & Maginitie (1965). The present study was made in order to ascertain if the anatomical and physiological properties of the compound eye are commensurate with those postulated by the model. The three properties studied were the inter-ommatidial angle, the receptive field of the retinula cells, and the relationship between light intensity and the magnitude of the generator potential. The angle between the axes of adjacent ommatidia was measured anatomically. This varies with the position in the eye but has a mean value of 3.9° in the horizontal plane and 2.4° in the vertical. During dark adaptation the secondary pigment cells contract by about 5 pm at either end, moving the pigment back away from the lenses and also exposing the focal region of the ommatidium. The receptive field of single retinula cells was measured electrophysiologically. The light flux curve for a single unit is a Gaussian function with a width at half the maximum light flux of 3.2° in the horizontal and 2.5° in the vertical plane when light adapted. After 15 min dark adaptation these increase to 8.5° and 4.5°. These values agree well with those predicted by the optomotor model. The increase in field width during dark adaptation should change the relative acuity for spatial wavelengths of striped patterns, and this also was found in the optomotor experiments. The relationship between the magnitude of the generator potential and the light intensity was logarithmic over the intensity range used.
