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Colourfulommatidiaofbutterflyeyes 1081 exists, but red-reflecting ommatidia are completely absent dorsally. Reflectance spectra The different reflection colours in Figs 2 and 45° 3 indicate that the ommatidia of butterfly eyes can be divided into distinct classes. This is confirmed by measurements of the reflectance spectra of individual ommatidia. Fig. 4 presents the spectra of two members of the two classes, yellow and red, distinguishable in the eye of Bicyclus anynana (see Fig. 2A). The reflectance spectra of the ommatidia within the same class appear to scatter slightly, but by no more than 5–10nm. The reflectance spectra peak at around 580 and 650nm and differ distinctly in shape. The reflectance of the yellow class covers a broad wavelength range, with a cut-off wavelength at approximately 600nm, whereas that of the red class is negligible at wavelengths below 560 nm and its cut-off is at approximately 700 nm. –15° Heterogeneity and regionalization of butterfly eye shine Butterfly eye shine can be observed from a large number of ommatidia using a setup that exploits the approximately spherical architecture of the compound eye (Figs 1–3). In conventional epi-illumination microscopy, background reflections usually obscure the eye –45° shine, mainly because the field diaphragm is Discussion imaged at the plane of observation. The image contrast is considerably improved when incident light is effectively channelled into the individual rhabdoms, which is achieved by careful diaphragming and focusing the light beam on the DPP. A further improvement is realized by diaphragming the image of the DPP. In this way, one can largely remove light scattered by pigments in the pigment cells and suppress the reflections from lens surfaces, e.g. from the corneal facet lenses and the optical components of the microscope. White 670 nm 550 nm Visualizing the eye shine with a large aperture gives an immediate impression of the distribution of the various classes of ommatidium over the eye. The striking feature of most butterfly eyes is the large degree of heterogeneity of the eye shine pattern. A survey of different species from the families Nymphalidae, Lycaenidae and Pieridae indicates that the eye shine emerging from individual facet lenses is characteristic of the species. The typical yellow/red pattern of Heliconinae (Fig. 2B) also exists in certain Nymphalinae (e.g. Euphaedra christyi), Charaxinae (e.g. Charaxes fulvescens) and Lycaenidae (e.g. Polyommatus icarus), but very different patterns also occur. Eye regionalization is apparent when a specialized dorsal area exists. Its extent can be large as in Bicyclus anynana (Fig. 2A), rather minor, as in Pieris rapae (Fig. 2C), or it can even be absent, as in Heliconius melpomene (Fig. 2B). In a comparative study of a number of heliconian species that all lacked a distinct dorsal area, we found that the ratio of the 15° Fig. 3. The eye of the small copper Lycaena phlaeas photographed from four dorsal to ventral directions, differing by 30° from each other, with broad-band, white light (halogen lamp) and monochromatic red (670nm) and green (550nm) light. The effective aperture of the objective is approximately 60 °. The frontal and ventral parts of the eye contain a mixture of red- and yellow-green-reflecting ommatidia, but dorsally the reflection colours are a mixture of blue and green; red reflection is absent from the dorsal region. The central ‘hot spot’ is due to reflection on the lens surfaces of the microscope objective. 0 ° is approximately horizontal.PDF Image | Reflections on colourful ommatidia of butterfly eyes
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