‘Mind-bending’ crystals color butterfly wings

YALE (US)—At the very heart of some of the most brilliant colors on the wings of butterflies lie bizarre structures that may be of use in harnessing the power of light.

The crystal nanostructures that ultimately give butterflies their color—called gyroids—are “mind-bendingly weird” three-dimensional curving structures that selectively scatter light, says Richard Prum, chair and the William Robertson Coe Professor in the Department of Ornithology, Ecology and Evolutionary Biology at Yale University.

Using an X-ray scattering technique at the Argonne National Laboratory in Illinois, Prum and colleagues determined the three-dimensional internal structure of scales in the wings of five butterfly species.

Details are published online in the Proceedings of the National Academy of Sciences.

The gyroid is made of chitin, the tough starchy material that forms the exterior of insects and crustaceans, usually deposited on the outer membranes of cells.

Prum wanted to know how a cell can sculpt itself into this form, which resembles a network of three-bladed boomerangs. They found that, essentially, the outer membranes of the butterfly wing scale cells grow and fold into the interior of the cells.

The membranes then form a double gyroid—or two, mirror-image networks shaped by the outer and inner cell membranes. The latter are easier to grow but are not as good at scattering light.

Chitin is then deposited in the outer gyroid to create a single solid crystal. The cell then dies, leaving behind the crystal nanostructures on the butterfly wing.

Photonic engineers are using gyroid shapes to try to create more efficient solar cells and, by mimicking nature, may be able to produce more efficient optical devices as well, Prum says.

More news from Yale University: http://opa.yale.edu/

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