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1082 D. G. Stavenga 1.0 0.5 0 Y ellow Red Fig. 4 can be neglected because the spectra were measured after repeated bright illumination so that the green rhodopsin was virtually fully bleached (Bernard, 1983). Prolonged dark adaptation, for a few hours, yielded a spectrum with much lower reflectance for the yellow class and with a slightly different shape (Stavenga et al., 2000a). In the case of the red ommatidia, reflectance is negligible below 560nm. This is probably due to a strongly short- wavelength-absorbing, red-transmitting screening pigment sequestered in certain photoreceptor cells near the rhabdom. In the yellow ommatidia, a similar, strongly short-wavelength- absorbing pigment is clearly absent because reflection is considerable at all wavelengths below 600nm. The long- wavelength cut-off values of the reflectance spectra, which are approximately 600 and 700nm for the yellow and red class, respectively, must be determined by the tapetal mirrors. In other words, the yellow-reflecting ommatidia have a tapetum that reflects up to approximately 600 nm and no screening pigment, and the red-reflecting ommatidia have a tapetum that reflects up to approximately 700nm together with a red photoreceptor screening pigment that absorbs up to approximately 560nm. Tapetum and screening pigments are expressed together in unique combinations, thus determining the ommatidial classes. The reflectance spectra measured from single facets in the eyes of several other butterfly species (Qiu et al., 2002; K. J. A. Vanhoutte and D. G. Stavenga, unpublished observations) suggest that this conclusion holds quite generally. Spectral shifts induced by red photoreceptor pigment The sensitivity spectrum of a photoreceptor cell that receives light filtered by a red screening pigment depends on the absorption spectrum of the visual pigment and that of the screening pigment and its effective density. A distinct red sensitivity, with spectra peaking even above 600 nm, has been noted in several butterfly species (Swihart and Gordon, 1971; Bernard, 1979; Steiner et al., 1987; Scherer and Kolb, 1987a). In principle, this sensitivity could be based exclusively on red- absorbing rhodopsins (Bernard, 1979). However, the longest peak wavelength of an insect rhodopsin determined so far is 600nm (Bernard, 1979; Bernard et al., 1988), and the often aberrant spectral shape of the sensitivity spectra indicates that red pigment filters play a central role in butterfly red sensitivity. A red filter can shift the sensitivity spectrum of a photoreceptor, which in the unfiltered situation peaks in the green or orange, towards longer wavelengths, i.e. into the red (Arikawa et al., 1999b). The spectral shift will be especially prominent in the basal photoreceptor, R9. This photoreceptor fills a short, basal part of the rhabdom, as has been demonstrated in Nymphalinae (Kolb, 1985), Papilionidae (Arikawa and Uchiyama, 1996) and Pieridae (Qiu et al., 2002). To investigate the effect of red screening pigments on the sensitivity spectrum of R9, I have made a simple computational model (Fig. 5). Fig. 5A treats the case of the red-reflecting ommatidia of Bicyclus anynana, where a red pigment filter is inferred. The transmittance spectrum of the red filter can be derived from the measured 400 500 600 Wavelength (nm) 700 Fig. 4. Reflectance spectra measured from single-facet lenses in the eye of the satyrine Bicyclus anynana. The spectra fall into two classes, yellow and red. The reflectance spectrum of the yellow class is broad, extends into the ultraviolet, is very minor above 600 nm and peaks at around 580 nm; the reflectance spectrum of the red class is a restricted band around 650nm and is negligible below 560nm. The spectra are normalized to the peak value of the yellow class. differently coloured facets can change markedly across the eye (M. Joron and D. G. Stavenga, unpublished observations), suggesting that heterogeneity and regionalization exist universally in butterfly eyes (Stavenga et al., 2001). Eye regionalization suggests that different eye areas have special functions (Bernard and Remington, 1991). A plausible interpretation for the function of a distinct dorsal eye area is that it is specialized for discriminating objects against the sky, where short-wavelength light is dominant (Stavenga, 1992). This explanation probably holds for several butterfly species, as is suggested by the commonly shorter wavelengths of the eye shine dorsally compared with the eye shine of the ventral areas of the eye (Stavenga et al., 2001). The usual absence of red-reflecting ommatidia from the dorsal eye area (Figs 2A,C, 3) also indicates that red sensitivity is not at a premium there. However, there is substantial interspecies diversity. For example, the reflection pattern of Hypolimnas anthedon (Nymphalinae) does not comply with the general rule of shorter-wavelength reflections dorsally because the eye shine in a large dorsal area is homogeneous yellow-orange and that in the ventral area is either yellow or variable-green. Also, electroretinogram recordings in Papilio xuthus suggest that short-wavelength sensitivity is prominent ventrally (Arikawa et al., 1987). Tapetum and photoreceptor screening pigment The reflectance spectra measured from single facets of the two classes in B. anynana show distinct differences (Fig. 4). How can these spectra be interpreted? In general, three main variables determine the reflectance: the tapetum, the visual pigments inside the rhabdom and the screening pigments in the photoreceptor cell, when granules containing the latter pigments occur in the immediate surroundings of the rhabdom. 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