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Abstract

1.

In male houseflies (Musca domestica) the frontal dorsal region of the eye contains a unique class of central rhabdomere (R7/8) not found in other eye regions or in female flies (Fig. 1). The rhabdomeres may be recognised in vivo by their red autofluorescence, and are called here 7r and 8r respectively.
2.

Difference spectra of 7r rhabdomeres, measured by microspectrophotometry of single rhabdomeres are indistinguishable from those of R1–6 (Fig. 2).
3.

Intracellular recordings coupled with dye injections have established that: a) 7r cells are indistinguishable from the peripheral photoreceptors R1–6, at least with respect to spectral, angular and absolute sensitivities, response waveform and noise characteristics (Figs. 4, 5; Table 1); b) 8r cells however are clearly distinguishable by virtue of their spectral sensitivity (Fig. 6), noise characteristics and response waveform (Fig. 5).
4.

Axonal profiles from cells stained intracellularly with the dye Lucifer yellow (Fig. 9) show that: a) 7r cells do not project to the medulla but terminate in the upper region of the lamina cartridge layer where they also project out one or more lateral branches; b) 8r cells project long axons through to the medulla.
5.

Electron microscopic examinations of cells initially identified by their autofluorescence indicate that 7r cells approximate many features of R1–6 cells, including cell body, rhabdomere and axonal diameters. In these respects 8r cells differ and show the characteristic morphology of other R7 and R8 cells (Fig. 8, Table 2).