The sensitivity of housefly photoreceptors in the mid-ultraviolet and the limits of the visible spectrum.

In experiments published in his book of 1882, Sir John Lubbock demonstrated that removal of the ultraviolet (u.v.) wavelength's decreased the effectiveness of natural sunlight in stimulating ants to carry their pupae into the dark. An apparently high sensitivity to near-ultraviolet light was subsequently shown in studies of phototaxis in bees (Bertholf, 1931a, b; Heintz, 1959) and flies (Bertholf, 1932; Cameron, 1938). More recently electrophysiological techniques have been employed to study selective adaptation of the retinal action potential (Walther & Dodt, 1959; Goldsmith, i960) and the spectral responses of single retinular cells (Autrum & Burkhardt, i960; Autrum & von Zwehl, 1962, 1964; Burkhardt, 1962; Autrum, 1965). This work demonstrates that honeybees (Apis) have u.v. receptors with peak sensitivity at 340 nm. and the cockroach (Periplaneta) at ~ 350 nm. The photoreceptors of flies (Calliphora) show two maxima, one of which lies at about 350 nm. The peaks are less well defined, but u.v. receptors maximal at about 350 nm. also seem to be present in Coleoptera, Lepidoptera (Hasselmann, 1962) and probably Odonata (Ruck, 1965). These peaks are not due to secondary excitation of the visual pigments through fluorescence of accessory pigment for (1) both bees and flies are phototactically several times more sensitive to the 365 nm. mercury line than to any longer wavelengths (Bertholf, 1931a, b, 1932; Cameron, 1938); (2) in flies the u.v. maxima seen in the retinal action potential are higher than the peaks in the visible region of the spectrum (Walther & Dodt, 1959; Burkhardt, 1962); and (3) bees can be trained to discriminate u.v. from all other wavelengths or mixtures of wavelengths (e.g. Daumer, 1956). The chromophore of the visual pigments of insects is the carotenoid derivative retinal (Goldsmith, 1958; Goldsmith & Warner, 1962, 1964; Goldsmith, Barker & Cohen, 1964; Goldsmith & Fernandez, 19666). There are as yet no absorption measurements of the u.v.-sensitive pigments, and in the absence of such data action spectra provide important clues to the pigment's molecular structure. Most of the behavioural experiments intended to bear on this matter have not resulted in proper action spectra (cf. Goldsmith, 1961). Although many of the electrophysiological determinations of spectral sensitivity made during the past decade obviate this criticism, none extends to wavelengths significantly shorter than 300 nm.* Possible sensitivity to u.v. light of shorter wavelength remains an open question, for Bertholf's (1932) curve of 'relative stimulative efficiency' for the phototaxis of Drosophila shows a minor maximum at