Rutowski, R.L. et al. 2005. Pterin pigments amplify iridescent ultraviolet signal in males of the orange sulphur butterfly, Colias eurytheme. Proc R Soc B. 272, 2329-2335.

Butterflies have a large array of nanostructural color production mechanisms. This paper focuses on the interplay of pigment and structure. Researchers studied the orange sulphur butterfly (Colias eurytheme). The dorsal side of the wings is yellow/orange from the pigment pterin while in males there is also iridescent ultraviolet (UV) coloration that is directional and produced by nanostructural lamellae arranged like shingles in parallel ridges of scales. These orange/UV scales are surrounded by brown scales containing melanin. The presence of pterins in the same scales that produce UV colors is an oddity as pterins absorb UV light.

To study this question researchers took parts of male butterfly wings, treating one by extracting pterins with NH₄OH and leaving the other untreated. The NH₄OH removes the pterins but leaves the structural component that generates the iridescent UV color intact.

The researchers used a spectrometer to detect the reflectance spectra of these wings at viewing angles where UV reflectance was at a maximum (UV+) and a minimum (UV-). Untreated wings exhibited two reflectance peaks in the UV+ orientation one at 340 nm (UV) and another at 620 nm (orange). At the UV- angle there was a similar spectrum but with little reflectance in the UV range. Extraction altered reflectance across all wavelengths. In the UV+ configuration UV reflectance increased 20%, in the UV- configuration it increased 27%. Hue angle and chroma were analyzed as well: hue angle increased in both orientations (reflections were more blue) and chroma decreased in both orientations (reflections became paler) after pterins were removed.

This evidence indicates that coloration pigments and structures are interacting to enhance coloration. Pterins appear to amplify the iridescent properties of the UV signals by making the difference in UV reflectance brightness between the UV+ and UV- orientations much greater. Without pterins there is a three-fold difference in UV intensity between the two configurations, with the pterins there is a twenty-fold change in UV brightness. These butterflies are believed to have long and short wavelength photoreceptors so color combination (UV and orange) is thought to contrast strongly with the background of green foliage, which absorbs UV light. Finally, small motions in the wings can create a large change in UV reflectance (from 3% to 63%) thanks to filtering by the pterin pigmentation. This great change in UV reflectance created as a male butterfly flaps his wings may be designed to take advantage of the fact that flashing signals draw strong responses in many species of butterfly. Researchers believe that the production of the UV signal is costly, which may indicate that it is part of an honest signal of fitness by males.

In this paper spectrometry was once again used to analyze color reflectance by animal structures. Chroma and hue, two concepts discussed in class, were also used a way to quantitatively analyze the changes in color reflectance by the treated and untreated butterfly wings.

As with the last paper, this paper also discussed the interaction of pigments and structural color production, showing that these two methods of producing colors can occur together in two different organisms. In this case thin-film interference was used by lamellae in the butterfly wings to produce iridescent UV reflectance. Also discussed in class is that different organisms have photoreceptors that have difference wavelengths of maximum absorbance. This paper discussed the fact that the UV and orange colors produced by the wings are particularly well detected by the photoreceptors of these butterflies, particularly in an environment of green foliage. If an organism wants to be seen it will exhibit colors different from its surroundings, a concept brought up in class which seems to be involved in the selection of these signaling colors.

Finally, mentioned in class was the importance of temporal characteristics of color. This seems to be the case here as flashing is another property of light that these butterflies can manipulate, possibly so that males can attract the attention of females. The importance of a flashing motion is likely related to the fact that, as we discussed in class, rhabdomeric eyes such as those used by insects (including butterflies) have better temporal discrimination than ciliary eyes such as those used by humans.