THE ancient Egyptians worshipped it and strove to duplicate its iridescent colours. Now physicists have laid bare the secret of the scarab beetle鈥檚 unusually vibrant shells.
Gymnopleurus virens beetles have shells that change from red in the centre to green around the edges or from green to blue. To understand why, John Brink of the University of Pretoria in South Africa and his colleagues examined the shell under an electron microscope. They found that the shells are made of thousands of ultrathin layers, with each successive layer slightly twisted in relation to the one above. 鈥淚t鈥檚 a corkscrew effect,鈥� says Brink.
This corkscrew structure causes the shell to reflect only that portion of light which has the same corkscrew orientation 鈥� known as circularly polarised light. 鈥淲hen the corkscrews match, you get astonishingly efficient reflection of almost 100 per cent,鈥� says Brink.
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The team also found that the shells have defects, in which a layer swings around by 90 degrees. This in turn changes the spacing between the layers, allowing the shell to reflect more than one wavelength of light. These defects combine with the shell鈥檚 shape to give it its iridescence.
鈥淚t鈥檚 actually flaws and mistakes in the shell鈥檚 arrangement that lead to such beautiful colours appearing,鈥� says Brink (Journal of Physics D, vol 40, p 2189).
