Visual system of the stalk-eyed fly, Cyrtodiopsis quinqueguttata (Diopsidae, Diptera): an anatomical investigation of unusual eyes.

Diopsid flies have eye stalks up to a centimeter in length, displacing the retina laterally from the rest of the head. This bizarre condition, called hypercephaly, is rare, but has evolved independently among several insect orders and is most common in flies (Diptera). Earlier studies of geometrical optics and behavior have led to various hypotheses about possible adaptive advantages of eye stalks, such as enhanced stereoscopic vision while other hypothesis suggest that eye stalks are an outcome of sexual selection. Here, we focus on how these curious distortions of head/eye morphology are accompanied by changes in the neural organization of the visual system of Cyrtodiopsis quinqueguttata. Histological examinations reveal that the optic lobes, lamina (La), medulla (Me), lobula (Lo), and lobula plate (LP) are contained entirely within the fly's eye bulbs, which are located at the distal ends of the eye stalks. We report that the organization of the peripheral visual system (La and Me) is similar to that of other Diptera (e.g., Musca and Drosophila), but deeper visual areas (Lo and LP) have been more strongly modified. For example, in both the lobula and lobula plate, fewer but larger giant collector neurons are found. The most pronounced difference is the reduction in the number of wide-field vertical cells of the lobula plate, where there are only four relatively large fibers, as opposed to 11 in Musca. The "fewer but larger" neural organization may enhance the conduction velocities of these cells, but may result in a loss of spatial resolution. At the base of the eye bulb, axon bundles collect and form a long optic nerve that extends the length of the eye stalk. We suggest that this organization of the diopsid visual system provides evidence for the costs of possessing long eye stalks.