Le contrôle automatique du flux lumineux dans l'oeil composé des Diptères

In the compound eye of the fly Musca, tiny pigment granules move within the cytoplasm of receptor cells Nos. 1–6 and cluster along the wall of the rhabdomeres under light adaptation, thus attenuating the light flux to which the visual pigment is exposed (Kirschfeld and Franceschini, 1969). Two recently developed optical methods (the neutralization of the cornea and the deep pseudopupil) combined with antidromic and orthodromic illumination of the eye (Fig. 1) make it possible to analyse the properties of the mechanism at the level of the single cell, in live and intact insects (Drosophila and Musca). The mechanism is shown to be an efficient attenuator in the spectral range (blue-green) where cells Nos. 1–6 have been reported to be maximally sensitive (Figs. 4c and d, 5b and 11b). In spite of the fact that the granules do not penetrate into the rhabdomere, the attenuation spectrum they bring about closely matches the absorption spectrum of the substance of which they are composed (ommochrome pigment, dotted curve in Fig. 11b). The dramatic increase in reflectance of the receptors after light adaptation (Figs. 3, 4b, 5a and 11a) can be explained as a mere by-product of the high absorption index of the ommochrome pigment, especially if one takes into account the phenomenon of anomalous dispersion (Chapter 8). The vivid green or yellow colour of the rhabdomeres would thus have a physical origin comparable to a metallic glint. Contrasting with the lens eye in which the pupillary mechanism is a common attenuator for both receptor types (rods and cones), the compound eye of higher Diptera is equiped with two types of “pupils” adapted respectively to both visual subsystems. A scotopic pupil is present in each of the six cells (Nos. 1–6) whose signals are gathered in a common cartridge of the first optic ganglion. This pupil comes into play at a moderate luminance (0,3 cd/m2 in Drosophila; 3 to 10 cd/m2 in Musca. Figs 13, 14, 15, 16). A photopic pupil is present in the central cell No. 7 whose signal reaches one column of the second optic ganglion. Attenuating the light flux for both central cells 7 and 8, the photopic pupil has its threshold about two decades higher than the scotopic pupil, just at the point where the latter reaches saturation (Fig. 3b, e-State II of Figs. 6b and 15). The photopic pupil itself saturates at a luminance one to two decades higher still (Fig. 3c, f=State III of Figs. 6c and 15). The two-decades-shift in threshold of these pupil-mechanisms supports the view that receptors 1–6 are a scotopic subsystem, receptors 7 and 8 a photopic subsystem of the dipteran eye. The luminance-threshold of the scotopic pupil (as determined with the apparatus described in Fig. 2) appears to be located at least 3.5 decades (Drosophila) or even 5 decades (Musca) higher than the absolute threshold of movement perception (Fig. 16). After a long period (1 hr) of darkness a light step of high intensity can close the scotopic pupil within about 10 sec (time constant τ≃2 sec as in Fig. 9) and the photopic pupil within no less than 30–60 sec. Some mutants of Drosophila possess only a scotopic pupil (wα, Figs. 4 and 5) whereas ommochrome deficient mutants lack both types of pupil (v, cn, see Fig. 7c, d). Comparable reflectance changes, accomplished within about 60 sec of light adaptation, are described for two insects having fused rhabdomes: the bee and the locust (Fig. 17).