Spectral sensitivity of dipteran retinula cells

Summary

Dipteran rhabdomeres are dielectric waveguides. Using dielectric waveguide theory an expression for the spectral sensitivity of retinula cells is derived. The spectral sensitivity of a retinula cell, S (A), depends strongly on the physical properties of the rhabdomere (diameter and refractive index) in addition to the spectral absorption of the photopigments. By correlating theory with existing experimental results we conclude that:

• 1. The effect of containing photopigment within a rhabdom of small diameter is (a) to shift the visible absorption peak to lower wavelengths and (b) to increase the UV peak absorption relative to the visible peak. The smaller the diameter of the rhabdom the greater the effect (Fig. 8 and 9).

• 2. The theoretical results (Fig. 10) for the spectral sensitivity,S (λ), of fly retinula cells are consistent with the single cell electrophysiology of Autrum and Burkhardt (1961) and Burkhardt (1962). The blue receptor type recorded by single cell electrophysiology is identified with rhabdomere number 7. The blue receptor is different from the green receptor type (rhabdomeres 1–6) because it has a smaller diameter not because of a different photopigment. The enormous measured UV sensitivity of the blue receptor type is due mainly to the waveguide effects caused by the small diameter of rhabdomere 7. Nevertheless, we conclude that in addition to a rhodopsin-like visible photopigment a separate UV photopigment is present in each rhabdomere. The spectral sensitivity of the photopigments consistent with these findings is shown as the dotted line in Fig. 10.

• 3. Rhabdomere 7 acts like a UV colour filter for rhabdomere 8, since it is more distal and has a large UV absorption. Thus the yellow-green receptor type measured by single cell electrophysiology, which has a low UV sensitivity compared with the blue type, may be retinula cell 8.

• 4. Spectral sensitivity curves inferred by optomotor reactions cannot be identified with retinula cells 1–6 in isolation of cells 7 and 8 and vice versa. Micro-spectrophotometric determination ofS (λ) is subject to methodological errors which render its reliability suspect. In the author's opinion, the intracellular recordings of Autrum and Burkhardt (1961) and Burkhardt (1962) represent the correctS (λ) curves for fly retinula cells.