A new kind of neural superposition eye: the compound eye of male Bibionidae

Abstract

IN neural superposition eyes the signals of receptors pointing to one region in space are neurally pooled. The optical and neural organisation of these eyes yields an enhanced absolute light sensitivity while minimising the loss of visual acuity that accompanies pooling. In the compound eyes of flies (Diptera), this is achieved in the following way. The distal endings of seven retinula cells lie near the back focal plane of each lens¹. Their light-sensitive structures, the rhabdomeres, are optically isolated from each other. The rhabdomeres of one ommatidium have different optical axes. The optical axes of seven rhabdomeres in seven adjacent ommatidia are parallel (Fig. 1a, ref. 2). The axons of such retinula cells (except the central ones, which pass through to the second optic ganglion) converge onto the same cartridge of secondary neurones in the first optic ganglion, the lamina^3,4. Thus, each lamina cartridge ‘looks at’ one point in space. In one suborder of flies (Cyclorrhapha), to which the housefly Musca and the fruitfly Drosophila belong, the arrangement of rhabdomeres is asymmetrical (Fig. 1a). Eyes organised in this way are one form of neural superposition eyes². Here, I provide evidence that a different kind of neural super-position eye exists in the Bibionidae, a family of flies of the suborder Nematocera. Bibionids are peculiar in being extremely sexually dimorphic in their visual system: the males have, in addition to small ‘female’ eyes, a pair of large dorsal compound eyes, functionally correlated presumably with the male habit of swarming and chasing females. These dorsal eyes differ from the eyes described in more advanced flies in two respects: the rhabdomeres are symmetrically arranged and the rhabdomeres with parallel optical axes are found in ommatidia √3 times further apart than adjacent ommatidia. The course of the retinula cell axons in the lamina differs accordingly.