Sexual dimorphism in the visual system of flies: The compound eyes and neural superposition in bibionidae (Diptera)

Abstract

1. Micro-optical and neuro-anatomical techniques were used to study the retinal organisation of the compound eyes and the receptor terminals in the lamina in Bibionidae (Diptera). 2. Male bibionids have divided compound eyes (Fig. 1). The dorsal eyes have larger facets and longer rhabdomeres than the ventral eyes and the eyes of females (Table 1). Groups of six peripheral retinula cells in six next-but-one ommatidia have parallel optical axes and their terminals in the lamina are shown to pass to next-but-one cartridges (Fig. 6, 7). 3. The retinal organisation of ventral eyes and the eyes of females differs from the one seen in the dorsal eyes of males (Figs. 2, 3). InBibio marci, the rhabdomeres in ventral and female eyes are arranged with the same radial symmetry as in the dorsal eyes, but interrhabdomere angles are smaller than would be expected for ideal neural superposition eyes (Fig. 5d). 4. In the ventral and female eyes ofDilophus febrilis, peripheral rhabdomeres are arranged in a horse-shoe like fashion around the central rhabdomeres (Fig. 2). Groups of four peripheral rhabdomeres in four neighbouring ommatidia have parallel visual axes (Fig. 5b). 5. The terminals of peripheral retinula cells in the ventral and female eye lamina pass to neighbouring and to next-but-one cartridges (Fig. 6). Long visual fibres, originating from the central retinula cells form en passant terminals in the ventral and female lamina. 6. It is shown that in eyes with open rhabdoms where the visual axes are not parallel, neural superposition reduces the amount of overlap between the visual fields of lamina cartridges provided the interrhabdomere angles are larger than 0.876 times the interommatidial angles (Fig. 8). 7. It is suggested that compared with the ventral and female eyes, the dorsal eyes of males are designed to increase the distance at which a small object can be detected against a homogeneous background (Fig. 9).