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Abstract:
1.
Intracellular recordings have been made from over 100 central retinula cells (R7 and R8) in the fliesCalliphora stygia (wild type),Calliphora erythrocephala (chalky) andMusca domestica (white).
2.
Spectral sensitivities determined with axial and non-axial illumination (Figs. 2, 3, 7, 9), polarisation sensitivities and their dependence upon stimulus wavelength (Table 2, Figs. 5, 6), and the effects of intense chromatic adaptation (Fig. 4) are described.
3.
All retinula cells R7 investigated had a major peak of spectral sensitivity in the ultraviolet. One class with a peak at ca. 340 nm and less than 10% sensitivity remaining at wavelengths longer than 400 nm, are termed UV cells. Another class with a peak at ca. 360 nm and a long tail of sensitivity (>10%) extending to 500 nm, are termed UT cells (Fig. 2).
4.
The ratio of UT cells to UV cells was approximately 7∶3, this is similar to the ratio between the two classes of distal central rhabdomeres, seven yellow (7y) and seven pale (7p) (after Kirschfeld and Franceschini, 1977) (Table 1).
5.
These results, in combination with recent microspectrophotometrical data (Kirschfeld et al., 1978; Kirschfeld, 1979) support the following interpretations:
a)
UV cells possess 7p rhabdomeres which contain an ultraviolet-absorbing rhodopsin that can be reversibly photoisomerised to a blue-absorbing metarhodopsin.
b)
UT cells possess 7y rhabdomeres and contain a different photopigment system to that found in UV (=7p) cells (Kirschfeld, 1979). Possibly this consists of a blue-absorbing rhodopsin and an ultraviolet sensitising pigment. The blue-absorbingphotostable pigment found in 7y rhabdomeres (Kirschfeld et al., 1978) has a screening function and modifies both spectral and polarisation sensitivity in UT cells.
6.
The R8 cells encountered probably all had rhabdomeres lying beneath 7y rhabdomeres; their spectral sensitivity could be accurately modelled by the screening action of the pigments present in 7y rhabdomeres upon a visual pigment with a peak absorbance at ca. 520 nm (Fig. 9).
