17 Jun 2002
The spectacular iridescence of butterfly wings has captivated many a scientist. Over 70 years ago, the bright colour of particular species' wings was attributed to thin-film interference effects rather than pigmentation. Multilayer mirrors recently-developed by 3M that defy Brewster's Law (OLE May p5) were inspired by the blue-winged Morpho butterfly.
Recently, however, two schools of thought on the role of the iridescent wings to the butterflies themselves have emerged. First, it was claimed that they acted as solar collectors. Now, the latest theory suggests that the opposite may be true, and that the wings actually act as sun blocks. Daniel Koon and Andrew Crawford, based at St Lawrence University in New York, measured the visible reflectance spectra of iridescent wing sections from a three different butterfly and moth species..
Using a ping pong ball for an integrating sphere, a fibre bundle as a light source and an EG&G spectrometer, they measured the reflectance of whole wings, both with and without the iridescence suppressed by an index-matching fluid. As the index-matching fluid seeps into the layers of air that separate the thin films of chitin, it destroys the iridescence, and the wings turn much darker in colour or even black..
Koon found that the iridescent effect reduces solar absorption of the wing by up to 20%. "Our results are more definitive than previous efforts, because they are based on actual data rather than numerical models, and also because we compared iridescent wings to those with iridescence removed," Koon told Opto and Laser Europe magazine..
Studies on green iridescent butterfly wings by Pete Vukusic of the UK's University of Exeter showed that the colour comes from mixing of yellow and blue colouration from different parts of the same scales that the human eye cannot resolve (the same basis as colour television)..
Exactly what benefits butterflies gain from their iridescent wings, solar blockers or not, remains a mystery. Koon believes that advantages in camouflage and courtship could outweigh the reduction of solar energy intake..
Story courtesy of Opto and Laser Europe magazine