Extraretinal pigmentation and colour discrimination. I. Choice of colour of substrate during oviposition in Drosophila melanogaster.

From the first demonstration that insects are capable of discriminating colours, obtained by Frisch with bees in 1915, a vast amount of information has been accumulated in the past fifty years. The first experimental methods, which permitted a general investigation of insect response to stimulation by coloured objects or by monochromatic light ('behaviour experiments') have been gradually replaced by more precise electrophysiological methods with which it has been possible to obtain information on the colour sensibility of even the individual retinal receptor (Burkhardt, 1964). The role of the non-retinal pigments, abundantly present in the compound eyes of insects, constitutes a particular aspect of colour vision, It is now known, in fact, that the principal function of such pigments is to screen the individual ommatidia laterally so that light rays cannot enter obliquely. In this way the pigmentation ensures the optical isolation of each ommatidium, which usually improves visual acuity and, especially in diurnal insects, facilitates apposition of the image. It seems to be established by now that the non-retinal pigments also enter into colour vision by exercising selective absorption of light rays of certain wavelengths. In D. melanogaster the ommatidium is surrounded by cells normally containing granules of two kinds: the red pigment, pteridine; and the brown pigment, homochrome, each of which presents a characteristic absorption spectrum. Furthermore, in some species numerous mutants for eye colour are known, which differ quantitatively one from another in respect of the brown or red pigment; indeed in a few cases there is complete absence of one or both pigments through a block in biosynthesis. Maas (1948) was the first to advance the hypothesis that the various eye pigments of Drosophila might function as colour filters and therefore might be said to be' selective'. Experimental demonstration of this view is credited to Fingerman (1952) who, measuring the intensity of the phototactic response to monochromatic light ranging from 366 to 700 m/i, obtained different curves for phenotypes differing in eye colour. Upon comparing the phototactic response of the brown mutant with the absorption spectrum of the sole pigment present in this mutant—that is, brown—Fingerman observed a marked coincidence, and he found the same to be true of the mutant