Fig. 1.

Characteristics of the dorsal rim area (DRA) of the compound eye. a Position and extension of the DRA. Scanning electron micrograph of the left eye of a monarch butterfly (dorsal and slightly ipsilateral view). The DRA (dark gray area) comprises approximately 100 ommatidia and is 2–3 ommatidia wide and approximately 45 ommatidia long. The midpoint of the DRA is at the dorsal origin of the horizontal rows of facets (circle), which is located lateral to the antenna base (DA unspecialized dorsal area). Bar 0.5 mm. b, c Orientation of rhabdom transverse axes (long axes of rectangular rhabdom cross sections). The rhabdom transverse axis indicates the microvilli orientation of the H receptors (cf. Fig. 2b, e). b Graphic representation of rhabdom transverse axes (ordinate) as a function of position along the DRA (abscissa). Rhabdom transverse axes are relative to the dorsal eye margin (0°), whereas rhabdom position is relative to the center of the DRA (0 μm; positive numbers: anterior). The graph quantifies the observation (Fig. 3) that the rhabdom transverse axes and, thus, the microvilli orientation of the photoreceptors change in a fan-like fashion along the fronto-caudal axis of the DRA. Data are from a single tangential section through the dorsal-most part of the eye (185 μm below cornea surface) containing most (87) of the approximately 100 ommatidia of the DRA. c Graphic representation of rhabdom transverse axes in the eight identified ommatidia shown in Fig. 2c, f at different retinal levels. Rhabdom transverse axes (ordinate) are relative to the dorsal eye margin (0°), whereas positions along rhabdom (abscissa) are relative to the distal end of the rhabdom (0 μm). In each of the eight ommatidia (different symbols), the rhabdom transverse axis is fairly constant indicating that the microvilli are well aligned along the rhabdom. d Reflectance spectrum of a tapetum in the DRA (thin line high resolution, thick line smoothed [Lowess 0.1] spectrum)