The dioptric system of the eye of Cybister (Dytiscidae: Coleoptera).

Abstract

1. The compound eye of Cybister is anatomically similar to that of Dytiscus and Hydrophilus . 2. The cornea and crystalline cone in the compound eye of Cybister (Dytiscidae) are composed of layers of unequal refractive index. With the exception of the outer 10 µ m of the cornea (where they are horizontal) the layers are arranged concentrically around a region of highest refractive index on the axis. 3. The refractive index of the cornea decreases from the central layer (1.724) to the periphery (1.561). The corresponding values for the crystalline cone are 1.435 and 1.366. The refractive index of the area between cornea and cone is 1.343; that of the clear zone is 1.341, and that of the proximal rhabdom is 1.361. 4. Parallel rays entering a facet converge to a focal region which extends from the proximal part of the cornea to the distal part of the cone. Rays cross the clear zone in a direction which depends on the angle to the axis and position on the facet. Up to an angle of 32° to the axis the rays are mainly bent back into the quadrant of origin so that rays entering many facets converge to a second focal region beyond the clear zone. These findings are consistent with the report of a first inverted although fuzzy image in the cone and a second image (Exner 1891). 5. Ray diagrams were constructed for three different positions of the distal pigment. If the cone tip is completely exposed, the receptor acceptance angle is 46°. With the pigment in the typical dark-adapted position the field of view is 38° wide. A light-adapted ommatidium would have an admission function about 18° wide according to ray tracing, but this could be reduced by the properties of the crystalline tract down which the light must pass. 6. It is concluded from the ray tracing that acuity is poor, but summation across the clear zone could confer a high sensitivity for the dark-adapted eye.