The function of photostable pigments in fly photoreceptors.

The photoreceptors in the fly's ommatidia contain a bistable visual pigment, which can be shifted back and forth by means of light of appropriate wavelengths. The situation is complicated, however, by the presence of photostable pigments. One of them (located in rhabdomeres no. 1-6) absorbs in the UV, another one (in rhabdomeres no. 7y) in the blue spectral range. Such pigments act as (dichroic) colour filters that modify the spectral and polarisation sensitivity of the photoreceptors by means of absorption. It could be shown furthermore that such pigments can also act as sensitizing pigments that modify spectral sensitivities due to sensitization.