Geometrical optics of theNotonecta eye: Adaptations to optical environment and way of life
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- Schwind, R. J. Comp. Physiol. (1980) 140: 59. doi:10.1007/BF00613748
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Certain features of the optical geometry of theNotonecta dioptric apparatus were studied. The cornea consists of two homogeneous layers; in the distal layer the refractive index is high, and in the proximal layer it is low. The two are separated by a bell-shaped transition region. This aspherical zone has almost exactly the shape one would anticipate in a system corrected for spherical aberration (Fig. 10). The outer surfaces of the individual corneal lenses are only slightly convex; therefore there is little change in the position of the plane of focus in the eye when the animal leaves the water.
The ommatidia, perpendicular to the corneal surface in the central region, lie at progressively greater angles at positions further medial and lateral (Fig. 3); for this reason the whole compound eye has a broad visual field (Fig. 11) despite its slight curvature.
75% of the optical axes of all the ommatidia are in the binocular visual space.
Observation of the displacement of the pseudopupil during rotation of the animal about a transverse axis reveals two zones of high acuity (Fig. 5). An animal resting below the water surface looks horizontally through the water with one of these zones. The other high-acuity zone is very small and lies 43 °±3 ° further ventral, so that it aims at the water surface just beyond the edge of the totally reflecting zone. With these ommatidia the animal can see the space just above the water surface.
In the ventral part of the eye the lattice of the optical axes is arranged in such a way that the vertical extent of a small object is always imaged in the same number of ommatidia regardless of distance, when the object is in the range 4.5 to 1.5 cm and 1 to 0 cm in front of the animal in the plane of the water surface. The range 1.5 to 1 cm is viewed by the high-acuity zone with which the animal scans the surface.
The extent to which the properties of movement-sensitive interneurons (Schwind, 1978) are based on the measured gradients in the optical lattice is discussed.