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Butterfly wings polarize light

07 May 2003

A study published in Nature reveals that butterflies use polarized light to attract mates.

Butterflies use polarized light to attract mates, according to a study published in Nature. The team from Duke University in the US and the Smithsonian Tropical Research Institute in Panama say this is the first proven example of polarized light being used for mate recognition. (Nature 423 31)

The team, headed by Alison Sweeney from Duke University, studied the bright blue light reflected from the wings of Heliconius butterflies. "Thin-film interference filters built from intricate structures made of chitin on butterfly wing scales are known to cause the colors seen in butterflies," Sweeney told Optics.org.

According to Sweeney, the peak reflection wavelength is 490 nm and can be seen between 20 and 30 degrees with respect to the normal to the wing's surface.

The researchers realised the significance of the emission by placing female butterfly wings behind filters that removed the polarization but kept the color signals intact. They found that iridescent male H. cydno butterflies flew towards the female wings less often when the polarization signal was blocked. As a control, Sweeney's team also saw that non-iridescent H. melpomene butterflies showed the same preference for polarization-blocked or polarization-transmitting wings.

Sweeney says that the polarized light is generated by reflection from a smooth dielectric surface on the wings. "Not all iridescent colors are polarized and this possibly has to do with the number of angles at which these thin-film interference filters project from the scale surface," she explained. "The actual physics behind the production of these colors has not been solved for many species."

Sweeney thinks the butterflies use the polarization phenomenon as a signal in their tropical forest habitat. "A polarized signal for iridescent wings is easily recognizable against a relatively unpolarized background," she said. "We now hope to take more polarization difference photographs of other iridescent butterflies to see how widespread the phenomenon may be."

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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