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  • 1
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract 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 (w α, 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).
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  • 2
    ISSN: 1432-1017
    Keywords: Fly visual pigment ; sensitizing pigment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Many lines of evidence suggest that the ultraviolet (uv) sensitivity found in the most common photoreceptor class in the fly is due to a sensitizing pigment which transmits the energy of absorbed light quanta to the visual pigment (Kirschfeld et al. 1977). It is shown that the uv extinction of the rhabdomeres has a vibrational fine structure corresponding to that found in the receptors' spectral sensitivity (Gemperlein et al. 1980). The uv extinction is greatly reduced when flies are reared on a carotenoid-deficient diet, in which case the vibrational fine structure in sensitivity is also lost. Properties (extinction, fluorescence) of several groups of substances that could represent the sensitizing pigment are illustrated.
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  • 3
    ISSN: 1432-1106
    Keywords: Vision ; Motion detection ; Single-photoreceptor stimulation ; Single-neuron recordings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Microscopic illumination of two neighbouring photoreceptor cells within a single ommatidium induces a strong sequence-dependent response in a directionally selective, motion-sensitive neuron. The response is characterized by a strong facilitation in the preferred direction and a weaker inhibition in the reverse direction. The data suggest that for each direction of apparent movement the signal from an ON-OFF pathway is released into the neuron via a parametric control mechanism which is activated by an adjacent channel.
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  • 4
    ISSN: 1432-1017
    Keywords: Photoreceptors ; Photostable pigments ; Dichroism ; Antenna pigment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract In the majority of ommatidia of the fly, the membrane of the central rhabdomere contains — besides the rhodopsin — a photostable pigment. Due to its selective absorption in the blue spectral range, this pigment (possibly a carotene) could modify the spectral sensitivity of the central receptor cells. It furthermore may change the fluidity of the microvillus membrane and hence affect the alignment of rhodopsin molecules. Indirect evidence for a possible role of the photostable pigment as an “antenna”-pigment for rhodopsin is discussed.
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  • 5
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Summary In the compound eyes of the fruitflyDrosophila, the dioptric system of each ommatidium is able to form virtual images of the receptor terminals (rhabdomere tips) throughout the whole depth of the eye. It is shown (§ 3) that 3 characteristic superposition phenomena occur for images formed by distinct ommatidia (Figs. 3b and 5). The most remarkable superposition appears at the point where the optical axes of all ommatidia converge (center of curvature of the eye). At this level, highly magnified virtual and erect images of corresponding rhabdomeres are superimposed, giving rise to adeep pseudopupil (Fig. 9). Since in the ommatidia ofDrosophila the rhabdome shows a pattern of 7 distal endings (Fig. 8a), the resultingdeep pseudopupil consists of 7 light spots with a similar pattern (Figs. 8b, 7, 11). Conversely thedeep pseudopupil of compound eyes which have fused rhabdomes consists of a single light spot (Fig. 19). Such pseudopupils can be best observed either with antidromic or with orthodromic illumination of the eye, according to the specific transmission or reflection properties of the rhabdomes. Thedeep pseudopupil of Dipterans is not to be confused with thecorneal pseudopupil (Fig. 13 a) and especially not with thereduced corneal pseudopupil observed with a reduced aperture of the microscope (Fig. 13 b), in spite of the remarkable similarity of these phenomena regarding the asymmetry and the dimension of their pattern (comp. Figs. 7 and 13b). Thereduced corneal pseudopupil consists of 7 facets whereas thedeep pseudopupil consists of 7 virtual images of the receptor endings. From the results of Kirschfeld (1967), the appearance of areduced corneal pseudopupil like Fig. 13 b on the eye ofDrosophila proves that 7 receptors located in 7 neighbouring ommatidia look in the same direction in space (Fig. 14). The existence of such an optical arrangement favors the view that the eye ofDrosophila, like that ofMusca, belongs to the “neural superposition type”. A comparative study between thedeep pseudopupil and thereduced corneal pseudopupil leads to the following geometric relation, which is specific of theDrosophila eye and probably of all compound eyes of the “neural superposition type”: $$\frac{D}{e} = \frac{R}{{f'}},$$ , whereD is the diameter of a facet,e the distance between the centers of two neighbouring rhabdomere endings,R the radius of curvature of the eye, andf′ the focal length (in air) of a corneal lens. Other types of pseudopupils, commonly appearing as dark spots in compound eyes, are explained on a basis similar to thedeep pseudopupil of Drosophila (§5). In fact, the dioptric system of an ommatidium can give virtual images not only of its distal receptor endings but of the whole intensity distribution (i.e. the whole “luminous structure”) which is present in its internal focal plane. If this structure is simple, thedeep pseudopupil, resulting from superpositions of virtual images, is likewise simple (Figs. 16 and 17). If the “luminous structure” is complex, as for example in the eye of the butterflyVanessa (Fig. 18a schematized in Fig. 18c), then thedeep pseudopupil shows the same complexity (Fig. 18 b and d). In compound eyes which lack screening pigment between their crystalline cones, one can seesecondary pupils of the 1st and 2nd order as described by Exner. Again they may be explained by superpositions of virtual images in the depth of the eye, according to Fig. 20. Moreover, thedeep pseudopupil of the “optical superposition eye” may be due to the fact that the more distal converging system of an ommatidium forms virtual images not of the rhabdome endings themselves but of real images of these endings (Fig. 21). Although the phenomenon of thedeep pseudopupil is not perceived by the animal, it is of interest for the experimenter who can use it: 1) to study the light receptors easily in the eye of live and intact animals, 2) to measure the physiological divergence angle between adjoining ommatidia, 3) to study the movement of the visual axis and the retinomotor adaptation of the receptors, and 4) to stimulate simultaneously manycorresponding receptors belonging to different ommatidia. The advantages of thisin vivo technique are discussed in § 6.3.
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  • 6
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Summary Optical characteristics of the dioptric system in the ommatidia of Musca have been analysed by use of “antidromic illumination” of the eye. The results indicate that the distal endings of the rhabdomers terminate near the focal plane of the dioptrics and that the quality of the lens is high enough to resolve some details of their shape. — Using optical methods it has been possible to confirm directly that the optical axes of 7 individual rhabdomers from 7 different ommatidia all converge to a common point in the distant surroundings. This is a characteristic for compound eyes of the “neural superposition” type. — The results are discussed on the basis of the hypothesis that the Musca eye is composed of two functionally different subsystems: One system (D) with high absolute sensitivity and low spatial resolution consisting of the sense cells no. 1 to 6, and a second system (H) with high spatial resolution and low absolute sensitivity composed of cells no. 7 and 8.
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  • 7
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Summary In the ommatidia of Musca, the light flux transmitted by each one of the rhabdomeres of sense cells no. 1 to 6 decreases as a function of time if light falls onto these rhabdomeres. With a similar time course the light flux reflected from these rhabdomeres increases. These changes take place within a few seconds following illumination. The results have been established in the intact animal using changes in the appearance of the pseudopupil as indicator and also in surviving preparations of the eye with direct inspection of the rhabdomeres. The changes are interpreted as a consequence of interactions between pigment granules in the sense cells and electromagnetic fields induced outside the rhabdomeres by light travelling on the inside: In the dark adapted situation the granules are quite distant from the rhabdomeres, the interaction is negligible. During light adaptation the granules move close to the rhabdomeres, and as a consequence, total reflection of the light in the rhabdomere is frustrated. The relatively rapid changes in the optical characteristics of the rhabdomeres are explained by the fact that the distance, the granules have to move in order to switch from one condition to the other is in principle on the order of the wavelength of light. The results indicate, that the changes in the position of the granules are induced by the excitation of the respective sense cells themselves, for instance by the degree of their depolarisation. No interaction between the sense cells of one ommatidium nor between those of different ommatidia could be found. The function of the movement of the pigment granules is interpreted as a means to protect the sense cells no. 1 to 6 against strong illumination. — Movement of pigment granules is not induced in sense cells no. 7 and 8 with light intensities which give maximal response in sense cells no. 1 to 6.
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  • 8
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Summary Thanks to a technique of optical neutralisation associated with a transilluinination of the eye, it is possible to study the photoreceptor endings (rhabdomere tips) in the compound eye of live and intact Drosophilae. The success of the neutralisation process directly confirms the idea that the convergence of the dioptric system in each ommatidium is essentially due to the refraction at the corneal outer surface. The remarkable regularity of the asymmetrical receptor pattern throughout the eye (fig. 7) is of functional importance. The divergence angle between the optical axis of neighbouring receptors, and their farfield radiation pattern are shown to depend respectively on the spacing and the diameter of the rhabdomere distal endings (fig. 8). The tip of the centrally located rhabdomere number 7 (fig. 5) is found to have a smaller optical diameter than its six neighbours and the extinction spectrum of this rhabdomere is different from those of the other ones. Modal patterns are observed at the distal tip of the rhabdomeres (fig. 9), confirming the waveguide properties of these components. The eye of Drosophila is morphologically composed of two equal parts, dorsal and ventral, in which the rhabdomere patterns are symmetrical (fig. 7). Sporadic irregularities are found in the border between these two parts (fig. 10). Actually the main importance of this neutralisation technique lies in its possible applications. The simultaneous visualization of a lot of receptors, in transmitted light, allows a precise stimulation, in incident light, of single and known cells in the eye of live insects. This method combined with other in vivo techniques such as those using the phenomenons of “corneal pseudopupil” (Kirschfeld and Franceschini, 1968) and “deep pseudopupil” (Franceschini and Kirschfeld, 1971a) may simplify further studies regarding the nervous integration of visual stimuli in the facet eye.
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  • 9
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. In male houseflies (Musca domestica) the frontal dorsal region of the eye contains a unique class of central rhabdomere (R7/8) not found in other eye regions or in female flies (Fig. 1). The rhabdomeres may be recognised in vivo by their red autofluorescence, and are called here 7r and 8r respectively. 2. Difference spectra of 7r rhabdomeres, measured by microspectrophotometry of single rhabdomeres are indistinguishable from those of R1–6 (Fig. 2). 3. Intracellular recordings coupled with dye injections have established that: a) 7r cells are indistinguishable from the peripheral photoreceptors R1–6, at least with respect to spectral, angular and absolute sensitivities, response waveform and noise characteristics (Figs. 4, 5; Table 1); b) 8r cells however are clearly distinguishable by virtue of their spectral sensitivity (Fig. 6), noise characteristics and response waveform (Fig. 5). 4. Axonal profiles from cells stained intracellularly with the dye Lucifer yellow (Fig. 9) show that: a) 7r cells do not project to the medulla but terminate in the upper region of the lamina cartridge layer where they also project out one or more lateral branches; b) 8r cells project long axons through to the medulla. 5. Electron microscopic examinations of cells initially identified by their autofluorescence indicate that 7r cells approximate many features of R1–6 cells, including cell body, rhabdomere and axonal diameters. In these respects 8r cells differ and show the characteristic morphology of other R7 and R8 cells (Fig. 8, Table 2).
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  • 10
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The population of the centrally located rhabdomeres no. 7 in the ommatidia of flies (Musca, Calliphora, Drosophila) is inhomogeneous: approximately 2/3 of them contain — besides a photoisomerizable rhodopsin — a photostable pigment. Its extinction spectrum has a maximum at 460 nm and two shoulders at 430 and 485 nm respectively. Extinction is maximal for theE-vector perpendicular to the microvilli. Whereas the functional role of the photostable pigment for receptor 7 has still to be worked out, its functional consequence for receptors no. 8, which are located proximal to receptors 7, is obvious: it modifies their spectral sensitivity by selectively absorbing blue light. Due to this “screening”-effect, a shift of the maximal sensitivity of receptors no. 8 is predicted from 490 nm (maximal sensitivity of unscreened receptor 8, Harris et al., 1976) to 520 to 540 nm. This is in agreement with recent electrophysiological data (Hardie, 1977). The results show that spectral sensitivities of receptors no. 8, as determined by means of the ERG of white-eyed mutants or of mutants lacking receptor 7, do not represent the spectral sensitivities of most of these receptors in wild-type flies.
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