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1080 D. G. Stavenga The measured reflectance spectra can be formally interpreted by realizing that light emerging from an ommatidium has travelled twice through the length of the rhabdom while having been reflected at the tapetum. Or, the reflectance spectrum, Rr(λ), is given by: Rr(λ) = M(λ)Tr2(λ) , where λ is the wavelength of the light, M(λ) is the reflectance spectrum of the tapetal mirror and Tr(λ) is the (single pass) transmittance spectrum of the rhabdom (Stavenga et al., 1977). The transmittance of the rhabdom is affected by two components: transmitted light is absorbed both by the visual pigments in the rhabdom interior and by the screening pigments in the exterior medium surrounding the rhabdom. The measurements of Fig. 4 were performed after prolonged pre-illumination that bleached the long-wavelength (green) visual pigment. The absorption of the screening pigments then determines the rhabdom transmittance in the long-wavelength range, above 550 nm, because absorption by the visual pigment has become negligible there. Results Eye shine Fig. 2 presents the eye shine of two tropical butterflies, the satyrine Bicyclus anynana and the heliconian Heliconius melpomene, as well as that of the small white Pieris rapae, photographed with the apparatus depicted in Fig. 1. The frontal, i.e. forward-looking, areas were selected because the interommatidial angle in this area is smallest and, hence, the number of shining ommatidia captured by the objective aperture is largest (see Stavenga et al., 2001). In all three cases in Fig. 2, ommatidial heterogeneity is strikingly apparent, with the ommatidia reflecting predominantly yellow or red. The heterogeneity in the eye shine pattern in Bicyclus anynana is restricted to the ventral eye area (Fig. 2A); dorsally, only one type of reflection, that of the yellow-reflecting ommatidia, occurs. A survey of the eye of Heliconius melpomene (Fig. 2B) shows that two ommatidial types co-exist throughout the eye. The ommatidia in the majority of the eye of Pieris rapae are red or deep-red, the latter ommatidia appearing rather dark in Fig. 2C; the dorsal ommatidia reflect a mixture of yellows, and the ommatidia in a transition zone of approximately 15 rows between the dorsal and ventral areas reflect rather uniformly red (Fig. 2C). Another example of a butterfly with a distinct dorsal area is the small copper Lycaena phlaeas (Fig. 3). This eye was photographed from four directions, differing by 30° from each other, in a vertical plane. In the main fronto-ventral area, two classes of ommatidium can be distinguished, reflecting predominantly in the green or red. The two classes can be easily discriminated by using suitable monochromatic light, i.e. with wavelengths of 670 or 550 nm. In the dorsal area, a mixed population of bluish-green-reflecting ommatidia Fig. 2. The eye shine patterns in the eyes of the satyrine Bicyclus anynana (A), the heliconian Heliconius melpomene (B) and the small white Pieris rapae (B) observed with the large-aperture optical apparatus depicted in Fig. 1. The ommatidia in the three species reflect either predominantly yellow or predominantly red light. The red reflection is absent from a large dorsal area of the eye of Bicyclus anynana and from a small dorsal area of the eye of Pieris rapae; in Heliconius melpomene, both reflection types co-exist throughout the eye. The central ‘hot spot’ is due to reflection on the lens surfaces of the microscope objective. The dark areas in A and B are caused by specks of dust; the dark facets in C have a strong deep-red reflection. The scale bars, 300 μm in A–C, refer to the central part of the figures only because the optical apparatus suffers from slight barrel-type distortion.PDF Image | Reflections on colourful ommatidia of butterfly eyes
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