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1078 D. G. Stavenga Bernard, 1968). The tapetum is formed by a tracheole folded into a stack of layers, alternately consisting of air and cytoplasm, thus creating an interference reflection filter. Incident light that has travelled through the rhabdom without being absorbed is mirrored by the tapetum. The eye shine is the fraction of light also escaping absorption on its way back. In an early study, Ribi (1979a) compared the colour of the tapetal reflection as seen in eye slices, from which the photoreceptor layer had been removed, with the colour of the eye shine, i.e. with the tapetum in the intact eye. He found that tapetal reflection and eye shine colours were virtually identical in Nymphalidae, Satyridae and Lycaenidae, but not in Pieridae. In pierids, a major part of the eye exhibited a prominent red eye shine, whereas the colour of the tapetum with the retina removed was green-yellow. The anatomy of the eye of the small white Pieris rapae showed that the red eye shine is the result of the presence of a red screening pigment, which exists in four clusters near the rhabdom, where it selectively absorbs short-wavelength light propagating along the rhabdom (Ribi, 1979b). Evidently, the function of the pigment clusters in Pieris rapae is identical to that of the corresponding pigment clusters in Papilio xuthus, namely to suppress short- wavelength light, thereby shifting the sensitivity spectrum of the long-wavelength receptors into the red to produce a spectrum corresponding to sensitivity spectra measured electrophysiologically (Shimohigashi and Tominaga, 1991). Recent anatomical work has revealed that the red pigment of Pieris rapae eyes consists of two types of photoreceptor screening pigment, coloured red and deep-red; i.e. the four proximal photoreceptors of an ommatidium are either red- or deep-red-pigmented. The two different types of ommatidium are arranged in a random, heterogeneous pattern in the retina, which can be observed in vivo via the eye shine (Qiu et al., 2002). In an extensive comparative study of butterfly eye shine, we found that the colour of the light reflected from individual, neighbouring ommatidia often varies substantially in many species, testifying to the strong heterogeneity of butterfly eyes (Stavenga et al., 2001). Moreover, a substantial proportion of the ommatidia appeared to reflect in the red. As in the established case for the pierids, this red reflection is presumably the result of red pigment filtering the light flux in the rhabdoms, suggesting that the red-reflecting ommatidia contain photoreceptors with peak sensitivity in the red. Inspection of the eye shine is a very attractive method of rapidly surveying the distribution of red-reflecting ommatidia within the heterogeneous ommatidial lattice. However, classical epi-illumination microscopy has serious shortcomings because only low-power objectives with small apertures can be successfully applied. This paper demonstrates that these limitations can be largely overcome with a special apparatus that exploits the optical properties of the butterfly compound eye. The apparatus allows a large-aperture objective to be used so that the tapetal reflections of numerous ommatidia can be observed simultaneously. This approach will facilitate the charting of butterfly eyes and thus stimulate further understanding of eye regionalization and heterogeneity (Stavenga, 1992; Stavenga et al., 2001). In addition to presenting a few exemplary cases of butterfly eye shine, it is argued that the physiological functions of the tapetal mirrors and the screening pigments can be inferred from reflectance spectra measured from individual ommatidia. Materials and methods Animals The eye shine of a number of butterfly species was investigated with the apparatus depicted in Fig. 1. The satyrine Bicyclus anynana and the heliconian Heliconius melpomene (see Fig. 2) were obtained from cultures maintained by the Institute of Evolutionary Sciences, University of Leiden, the Netherlands. The small white Pieris rapae (see Fig. 2) and the small copper Lycaena phlaeas (see Fig. 3) were captured locally. Specimens were immobilized with wax and mounted on the platform of a goniometer. Apparatus The optical apparatus (Fig. 1) is, in principle, a modified epi- illumination microscope. The rationale of the instrument is that incident light applied to a butterfly eye is channelled by the facet lens and crystalline cone into the light-guiding rhabdom. Light reaching the ommatidial tapetum is reflected and guided back through the rhabdom (see Fig. 1 inset); when not absorbed there, it leaves the eye again and is then observable as the eye shine. Because butterfly eyes, like those of most insects, are locally more-or-less spherical, the visual axes of the ommatidia intersect at the eye’s centre of curvature. Hence, the optimal way to fill ommatidia with light is to position the eye’s centre at the focal point of an objective lens (L1 in Fig. 1) so that a point source at infinity is focused on the eye’s centre. The point source, which is in reality a slightly extended light source, is created by a white light source focused by lens L2 onto diaphragm D1, which is placed in the focal plane of lens L3. The parallel beam leaving L3 is mirrored by a semi- reflecting mirror, angled at 45 ° with respect to the optical axes of L1 and L3. L1 and L3 form a telescopic lens pair because they are confocal. The reflected light beams leaving the individual ommatidia diverge slightly, depending on the extent of the visual fields of the ommatidia. The beams intersect each other in the eye’s centre; the image created there is called the deep pseudopupil (DPP) (Franceschini and Kirschfeld, 1971). In the case of the butterfly, it is also called the luminous DPP (Stavenga, 1979). When the DPP is adjusted to the focal point of L1 and this point coincides with the centre of the goniometer, the eye shine in various areas of the eye can be rapidly scanned. Lens L4 is placed confocal with L1 and, hence, the telescopic lens pair L1 and L4 images the DPP in the back focal plane of L4, where diaphragm D2 is positioned. The eye shine at the level of the corneal facet lenses is finally imaged by lens L5, placed confocal with L4. The projected image is then photographed with a photomicroscope. To obtain an optimal picture, the areasPDF Image | Reflections on colourful ommatidia of butterfly eyes
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