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Holey wings. Natural photonic crystals give butterfly wings iridescent blue (top) or blue-violet (middle) color, but they're absent in brown-winged butterflies (bottom).

Showing Off vs. Staying Warm

Most male lycaenid butterflies display iridescent blues. In populations living at altitudes above 2000 meters, however, they've forsaken their Technicolor suits for a drab brown. A new study suggests that the dull coloring has a purpose more important than titillating potential mates: It helps the highlanders survive the mountain chill.

Butterflies rely solely on the sun's energy to bring their bodies up to working temperature, so heat management is crucial. That's why butterflies spread their wings and soak up as many rays as possible before starting their day. Biologists hypothesized that the brown lycaenid males were somehow able to capture the sun's radiation more efficiently, but they lacked conclusive evidence.

Scientists now report in the February issue of Physical Review E that they have found such evidence in the tiny scales that give the wings their color. In blue butterflies, these are made of nearly clear material. An array of submicrometer-sized holes arranged in a particular pattern reflect blue light, the scientists discovered by examining the wings with an electron microscope. This arrangement is similar to that found in so-called photonic crystals, which engineers use to trap and manipulate photons. No such holes were found in the scales of their brown brethren, says co-author Jean-Pol Vigneron, a physicist at the University Notre-Dame de la Paix in Namur, Belgium. Spectroscopic measurements demonstrated that more light bounced off the blue compared to the brown butterfly's scales, giving the lowlander its flashy looks and--in case of the highlander--letting much needed energy pass through to the wing below. Thermal analysis confirmed that under strong artificial light, brown wings soak up much more heat than blue ones.

The blue butterflies' scales might be a compromise, says Andrew Parker, a biologist at Oxford University, United Kingdom. The brown butterflies can survive the cold, but lack the flashy colors that presumably help attract mates. The blues look sharp but are confined to living in moderate climates. Parker says that scales reflecting red light may have all the advantages: color and good heating properties because they let more energetic blue light pass. But the photonic crystals would have to be considerably enlarged. For a red butterfly, "weight may become a problem," speculates Parker.

Related site
Jean-Pol Vigneron's site (in French)