Again, no effects of adaptation were observed. All the reflected light is contained in an angle of ± 5 degrees to the incident beam, although it enters and emerges via the large patch of facets mentioned above (6). When the eye is illuminated by a parallel beam, light is reflected back out of the eye. This also demonstrates the wide acceptance angle of the exposed ends of the rhabdom columns. This was directly demonstrated by recording from a retinula cell and stimulating the eye by a moveable slit of parallel rays. A parallel beam falling on the eye reaches a single receptor via a circular patch of facets subtending about 30 degrees at the centre of the eye. Optomotor experiments with stripes of differing widths show that the angular sensitivity of the receptors is fully utilized behaviourally, and that the eye functions in relatively dim light. The angular sensitivity curve of the receptors is 6 to 8 degrees wide at the 50% sensitivity contour. The acuity and origin of this image were investigated by several methods. A real erect image is formed on the receptor layer. No evidence of functional light guides crossing the clear zone could be found. The distal pigment remains between the cones even in daylight. Between the crystalline cones and the long rhabdom is a wide clear zone crossed by narrow extensions of the retinula cells. The anatomy of the eye is described in seven representative genera of Australian Hesperioidea.
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