Fig. 2. Anatomical and optical properties of the dorsal rim area of the eye. (A) Light micrograph of a tangential section through the dorsalmost part of the eye. Note the missing screening pigment and the enlarged, trapezoidal rhabdoms in the ommatidia of the dorsal rim area (DRA, top). The much smaller rhabdoms of the unspecialized dorsal ommatidia (DA, bottom) are densely surrounded by screening pigment. Scale bar, 10µm. (B) Three-dimensional representations of photoreceptor visual fields in the DRA (above) and in the unspecialized DA of the eye (below). The x and y directions are parallel and perpendicular, respectively, to the elongated DRA. (C) Comparison between visual field size and interommatidial angle of photoreceptors in the DRA. The T symbols indicate position and orientation of the retinulae within the dorsal rim area (compare enlarged cross-section through an ommatidum at the lower right; cells are numbered according to) (Burghause, 1979). Typically sized visual fields (in grey; acceptance angle 20°) of three adjacent ommatidia (in white) are overlaid on a schematic representation of a cross-section through part of the dorsal rim area. The angular distance between the rhabdoms (interommatidial angle ) is approximately 1°. Note that the visual fields overlap extensively. (A) After Labhart and Petzold, 1993; (B) after Labhart et al., 1984; (C) according to data by Blum and Labhart, 2000.