Fine structure of the compound eye of the black fly Simulium vittatum (Diptera: Simuliidae)

The fine structure of the ommatidia in light- and dark-adapted eyes of male and female Simulium vittatum Zetterstedt was investigated using scanning and transmission electron microscopy. The male eye is divided into distinct dorsal and ventral regions. The facets in the dorsal region are approximately two times larger than those in the ventral one, which are similar in size to the ones in the female eye. All ommatidia of S. vittatum examined consist of two general regions: a distal dioptric apparatus with bordering primary and accessory pigment and Semper cells, and a sensory receptor layer. Each ommatidium in the female eye and ventral eye of the male has eight retinular cells (R cells): six peripheral (R1–6) and two central (R7, R8). R7 occurs distally and R8 basally. Strikingly, the ommatidia in the dorsal eye of the male lack the R7 cell. In all ommatidia, rhabdomeres on the inner surface of the peripheral R cells are separate throughout their length, creating an open rhabdom. A greater diameter of corneal facets, elongated peripheral R cells, and perhaps the lack of the R7 cell are specializations of the dorsal region of the eye that help the male to detect small, rapidly moving females against the skylight as they fly above the swarm of males. Differences observed between light- and dark-adapted eyes of male and female S. vittatum were the same and were associated with the internal components of the peripheral R cells.