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Abstract

Photoreceptors of flies contain pigment granules which upon illumination of the receptors migrate towards the rhabdomere and act as a ‘longitudinal pupil’. Data in the literature concerning the effect of the pupil on the spectral sensitivity are contradictory. Therefore spectral sensitivity ofMusca photoreceptors upon light adaptation was reinvestigated.

The change in spectral sensitivity of fly photoreceptors upon light adaptation as measured by Hardie (1979) was confirmed. Taking into account waveguide optics this change was explained from absorbance spectra of pupillary granules, measured by microspectrophotometry in squash preparations. Furthermore the pupil absorbance spectrum determined in vivo (Stavenga et al. 1973) was interpreted. The absence of a change in spectral sensitivity upon light adaptation measured by pupillary reflexion (Bernard and Stavenga 1979) is explained by a local-triggering of the pupil.