The small structures in the scanning electron microscope image of a butterfly wing scale (a) are natural photonic crystals that give the wings of some butterflies their brilliant iridescent blue colors. The structures in the second image (b) are responsible for a blue-violet iridescence. In the third image (c), the small structures are almost entirely absent, and the butterfly wing scales are a dull brown shade. New research suggests that photonic crystals keep butterfly wings cooler, as well as making them beautiful. In higher elevations where butterflies are more reliant on sunlight to keep them warm, some of the insects have evolved wing scales in which the photonic crystals have been disrupted (as in image c), improving the chances that they survive long enough to mate despite the frigid climate.
Source: L. P. Biro et al., Physical Review E, February 2003
In recent years, scientists have discovered that the iridescence of various colorful creatures, from beetles to birds to butterflies, is often due to microscopic structures known as photonic crystals. Unlike pigments, which absorb or reflect certain frequencies of light as a result of their chemical composition, the way that photonic crystals reflect light is a function of their physical structure. That is, a material containing a periodic array of holes or bumps of a certain size may reflect blue light, for example, and absorb other colors even though the crystal material itself is entirely colorless. Because a crystal array looks slightly different from different angles (unlike pigments, which are the same from any angle), photonic crystals can lead to shifting shades of iridescent color that may help some animals attract mates or establish territories.
A collaboration of researchers from Hungary and Belgium (Jean-Pol Vigneron, Universitaires Notre-Dame de la Paix, Brussels, firstname.lastname@example.org, 011+32-81 724711) may have discovered why the males in certain populations of lycaenid butterflies carry the striking, photonic crystal coloration, and males in other lycaenid populations do not. The researchers examined butterfly scales through high-resolution scanning electron microscopes (see image), and confirmed that indeed the colorful butterflies’ scales included arrays of submicron-sized holes that formed natural photonic crystals. Their closely related brethren from higher elevations did not have the hole arrays in their scales, and their wings were dull brown rather than iridescent blue. The difference, it seems, may be due to a question of survival. The researchers found that the plain brown butterfly wings warmed much more than the iridescent blue wings when each were exposed to identical illumination. The researchers believe that the butterflies at high elevations trade flashy iridescence for light-absorbing brown so that they can withstand colder temperatures, and survive long enough to mate.
If photonic crystals can have such a dramatic impact on butterfly thermal management, suggest the researchers, manmade photonic crystals may someday provide flexible thermal protection in extreme environments, possibly being incorporated into such things as space suits or desert garments. (L. P. Biro et al, Physical Review E, February 2003; text www.aip.org/physnews/select )
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