The compound eye of the tsetse fly (Glossina morsitans morsitans and Glossina palpalis palpalis)

Abstract We have examined the retina of the tsetse fly Glossina morsitans and G. palpalis using anatomical, optical, biochemical and electrophysiological techniques. The eye is basically very similar to those of other higher Diptera such as Musca and Calliphora. The ommatidial organization has an open rhabdom arrangement typical of a neural superposition eye. The central rhabdomeres R7 and R8 are smaller in diameter than peripheral rhabdomeres (R1-6) except at the dorsal margin of the eye, where they are greatly enlarged. The number of secondary pigment cells is unusually large with 16–18 surrounding each ommatidium. The facet lenses are also unusually thick with a weakly curved outer surface and a strongly convex inner surface. It is shown how this gives rise to the characteristic striped reflections from the tsetse eye by total internal reflection, and possible functions for this are considered. As in most other dipterans, the visual pigment chromophore is 3-hydroxy retinal and an ultraviolet sensitizing pigment, 3-hydroxy retinol is present also. Photoreceptor cells R1-6 have a similar spectral sensitivity to those in Musca, although the position of the green peak (500 nm) is some 10 nm longer. Two spectral classes of R7 correspond to the so-called 7y and 7p cells in Musca, with predominantly ultraviolet sensitivity, and the spectral sensitivity of the R8 cells encountered resembles that of so-called 8y cells (λ max 520 nm). Due to a dietary deficiency, the eyes of flies raised on porcine blood contain no traces of C40 carotenoids. This is correlated with the observation that the spectral sensitivity of both 7y and 8y cells are systematically higher in the blue (400–500 nm) than their counterparts in Musca or Calliphora.