The primary aim of the experiments described in this article was to localize the origin of the complex fluorescence in the compound eye of flies. The eye tissue was dissected and the fluorescence from cells and cell organelles was recorded by microspectrofluorometry. Using this technique, fluorophore systems were detected in the rhabdomeres, Semper cells and corneal lenses. The fluorophore systems are photoreconvertible by UV and blue light. 2. The fluorophore systems in the rhabdomeres and Semper cells are similar. The intensity of the fluorescence from the microvilli is enhanced up to 29 X by adaptation to UV light. The enhancement is inversely related to the rhodopsin content in the microvilli, indicating that the chromophoric group of the fluorophore is not a vitamin A derivative. 3. The enhancement of the fluorescence by UV light strongly depends on pH, suggesting that the photoreconvertible fluorophore systems in the microvilli and Semper cells are photosensitive redox pigments. These redox systems are probably located in the membranes of the microvilli in the photoreceptors, and in the endoplasmic reticulum of the Semper cells, or they are coupled to filaments in the cytoskeleton of both cell types. 4. Preliminary reaction schemes for the photoreactions based on the recorded excitation and emission spectra and photokinetics were developed. A primary pigment in the microvillous structure, AR, or in organelles in the Semper cells, AS, is converted by UV light into an excited state AR* or AS*, which either relaxes to the primary pigment by photon emission, or converts into an intermediate X, which by proton uptake changes into stable products, BR or BS. Blue illumination converts BR and BS into the excited states BR* and BS*, which either relax by photon emission to BR or BS, or convert into an intermediate Y, which after deprotonation reconverts into the primary pigment AR or AS. 5. Estimation of the molecular density showed that the concentration of the fluorophore in the microvilli presumably is almost equal to maximal rhodopsin concentration. The high density suggests that the fluorophores have a specific function in transduction or adaptation of the visual process.
