The superposition eye of skipper butterflies

Abstract

1. The anatomy of the eye is described in seven representative genera of Australian Hesperioidea. Between the crystalline cones and the long rhabdom is a wide clear zone crossed by narrow extensions of the retinula cells. The distal pigment remains between the cones even in daylight. 2. No evidence of functional light guides crossing the clear zone could be found. 3. A real erect image is formed on the receptor layer. The acuity and origin of this image were investigated by several methods. 4. The angular sensitivity curve of the receptors is 6 to 8 degrees wide at the 50% sensitivity contour. 5. 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. Adaptation changes are small. 6. 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. This was directly demonstrated by recording from a retinula cell and stimulating the eye by a moveable slit of parallel rays. This also demonstrates the wide acceptance angle of the exposed ends of the rhabdom columns. 7. When the eye is illuminated by a parallel beam, light is reflected back out of the eye. All the reflected light is contained in an angle of $\pm $ 5 degrees to the incident beam, although it enters and emerges via the large patch of facets mentioned above (6). Again, no effects of adaptation were observed. 8. The relation between the direction in which a ray enters the optical system of the cornea and cone and the direction it leaves was measured directly by a rotatable microscope of narrow aperture. 9. The mechanism by which the optical system forms the erect image on the receptor layer was demonstrated by tracing rays through scale drawings of the components. To do this the refractive index was measured in all components. The corneal surface acting as a lens forms the first image within the cone. The crystalline cone is non-homogeneous and acts as second lens in each ommatidium. 10. The skipper eye therefore illustrates Exner's superposition principle, and it does so in daylight.