Fluorescent materials in insect eyes and their possible relationship to ultra-violet sensitivity

Abstract

It was suggested that insect eyes contain at least two light-sensitive pigment systems: one based on rhodopsin and the other on a non-rhodopsin complex. The rhodopsin system is responsible for sensitivity in the visible and for some sensitivity in the ultra-violet (u.v.), whereas the non-rhodopsin system functions only in the u.v. To support this hypothesis, experiments were conducted to find fluorescent substances in insect compound eyes which had excitation spectra compatible with the physiology of the insect eye in the u.v. In vivo fluorescence was demonstrated in flies, bees, and moths. Intensely u.v. fluorescent materials emitting in the visible were extracted in substantial quantities in all insect eyes examined. The excitation spectra of these materials were well within the insect u.v. sensitivity region and the u.v. excitation peak matched the u.v. eye sensitivity peak. Furthermore, histological sections of the eye revealed that the fluorescent material was present in the retinal cell. These results led to the proposal that the fluorescent material may appear in the insect eye in two forms; as a radiative system and as the chromophore of a non-radiative, non-rhodopsin, u.v.-sensitive pigment system. In the nonradiative system the chromophore is postulated to be present in a complex which directly couples the absorbed energy into a chemical system, whereas in the radiative system the chromophore is not part of this complex and the absorbed energy is dissipated by radiative means. The fluorescent materials were tentatively identified as hydroxy indole derivatives and its was suggested that they are absent in the mutant strains ‘chalky’ and ‘white-apricot’ of Calliphora erythrocephala since the eye pigment (ommochromes) missing from these mutants and hydroxy indoles are derived from a common precursor, tryptophan.