|Prepona praeneste buckleyana (male) in human visible light (left) and UV (right).|
Butterflies are known for their brilliant display colors, but did you know that these colors are even more complex than what we see? The tiny scales on butterfly wings are capable of reflecting ultraviolet light (UV), and their eyes are capable of detecting it with UV sensitive opsins.
By taking photographs of UV light, we can see some of these hidden patterns, and try to interpret the colors they might perceive. The image at the top of this post, for example, reveals a UV pattern across the middle portion of the wing. The picture below demonstrates how this UV can contribute to colors as the butterflies perceive them. The forewing is orange to our eyes, but it has bright UV reflectance as well, so this color would better be described as a UV-orange (although I’m sure UV sensitive organisms would come up with some better names if they could). The hindwing, however, is just yellow as we know it, with no UV.
|Hebomoia leucippus in human visible light (left) and UV (right).|
I’d also like to clarify that the UV colors of butterflies are the result of UV reflectance, which is something entirely different than fluorescence (where UV light seems to make an object glow, like clothes under a blacklight). I’ll make another post about fluorescence in the future, as there are some very interesting examples of biofluorescence in nature.
Want to learn more about how these colors are made? Butterfly scales are capable of producing structural coloration through interference mechanisms such as diffraction gratings. These typically reflect short wavelength light, such as the blue of Blue Morphos and UV in the butterflies above. The wing membrane below the scales may contain various pigments which also contribute to the color of the wing, and the interaction of structures and pigments increases the butterfly’s potential color gamut. SmarterEveryDay made a very interesting video about butterfly structural coloration, which I recommend watching.
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