The rhabdomeres of the visual cells in the blowfly Calliphora erythrocephala and the fruit fly Drosophila melanogaster are twisted along their long axes. In rhabdomeres of the visual cells R1-6 it is possible to distinguish 3 regions differing in twist rate. In the proximal and distal regions the twist is slight (e.g., 0.52°/μm) or absent, whereas in the middle the twist rate is high (e.g., 2.40°/μm). The twisting of the rhabdomeres of R1-3 is congruent and codirectional, and that of R4-6 is its mirror image. The significance of twisting with regard to the dichroic absorption of the microvilli and to the polarization sensitivity and the self-screening of R1-6 is discussed. In particular, it is shown that the dichroic absorption of a single microvillus of R1-6 must be greater than 2; it follows that the absorbing dipoles of the visual pigment molecules must be more or less parallel to the axes of the microvilli. Finally, it can be shown that the twisting of the rhabdomeres R1-6 prevents self-screening - despite high microvillar absorption. Because the microvilli are not uniformly oriented, the twisted rhabdomeres R1-6 are especially effective in absorbing unpolarized light. © 1979 Springer-Verlag.
