Abstract
Butterflies have well-developed color vision, presumably optimally tuned to the detection of conspecifics by their wing coloration. Here we investigated the pigmentary and structural basis of the wing colors in the Japanese yellow swallowtail butterfly, Papilio xuthus, applying spectrophotometry, scatterometry, light and electron microscopy, and optical modeling. The about flat lower lamina of the wing scales plays a crucial role in wing coloration. In the cream, orange and black scales, the lower lamina is a thin film with thickness characteristically depending on the scale type. The thin film acts as an interference reflector, causing a structural color that is spectrally filtered by the scale’s pigment. In the cream and orange scales, papiliochrome pigment is concentrated in the ridges and crossribs of the elaborate upper lamina. In the black scales the upper lamina contains melanin. The blue scales are unpigmented and their structure differs strongly from those of the pigmented scales. The distinct blue color is created by the combination of an optical multilayer in the lower lamina and a fine-structured upper lamina. The structural and pigmentary scale properties are spectrally closely related, suggesting that they are under genetic control of the same key enzymes. The wing reflectance spectra resulting from the tapestry of scales are well discriminable by the Papilio color vision system.
Highlights
Butterflies are highly visual animals, with well-developed color vision systems
Wing scale colors are due to spectrally tuned pigment filters and interference reflectors The wings of the Japanese yellow swallowtail P. xuthus butterfly are colored by scales that contain various pigments and consist of nanostructured elements (Figures 1 and 2)
It has been commonly assumed that the color of the cream and orange scales is principally caused by their specific papiliochromes, especially because the abwing and adwing side of the scales are colored (Figure 3A, B, column I), and the reflectance spectra measured from both scale sides differ only slightly (Figure 3A, B, column III)
Summary
Butterflies are highly visual animals, with well-developed color vision systems. The butterfly species whose visual capacities have so far been investigated into the most detail is the Japanese yellow swallowtail butterfly, Papilio xuthus. P. xuthus color vision appears to be tetrachromatic, based on a set of UV, blue, green and red receptors [3]. Butterflies use their color vision to search for flower nectar and presumably for detecting and recognizing conspecifics [2]. The lower lamina is a more or less flat plate, connected by trabeculae to the elaborate array of parallel ridges of the upper lamina, which are connected by crossribs The latter form open windows to the scale lumen, but when a membrane connects the crossribs, the windows are closed [5,6]
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