Green beetle's super-shiny secret 绿甲虫发亮的原因

The stunning¹ metallic² green sheen of the "jewelled beetle³" is produced by microscopic⁴ cells in its exoskeleton.

“宝石甲虫”发出漂亮金属绿光是因为它外骨骼的微观细胞。

Under left circularly polarised light, the same beetle looks "super-green"

Researchers reporting in the journal Science have revealed that, with no green pigments⁵, these structures make the beetle appear iridescent⁶（虹彩色的） green.

These cells, they said, were almost identical to hi-tech liquid crystals.

The scientists found that the structures make the beetle appear very intensely green under light that is polarised（极化） in one direction.

Polarisation can be thought of as the orientation⁷ of light waves. And white light, or natural light, is essentially⁸ a mixture of randomly⁹ polarised light.

The structures in the jewelled beetle's (Chrysina gloriosa's) exoskeleton（外骨骼） are spiral（螺旋形的） - or helical - so they reflect light that is polarised in the same direction as that spiral.

Without any need for pigments, these "photonic（光子）" structures manipulate light to make the beetle appear bright green.

Lead author Mohan Srinivasarao, from the Georgia Institute of Technology in Atlanta, had previously¹⁰ studied cholesteric（胆甾相液晶） liquid crystals, which are used in hi-tech reflective displays.

"The two systems were remarkably¹¹ similar," he told BBC News. "When I first looked at the beetle's exoskeleton (with the microscope) I thought, 'I've seen this somewhere before'."

Light trick

Having studied the exoskeleton using a highly sensitive confocal microscope, he and his team reconstructed a 3D map of its underlying¹² structure.

They discovered that the green colour is produced by the pitch, or size, of the helices, as the distance over which the helix turns 360 degrees controls what colour of light it reflects.

Dr Srinivasarao explained that because this pitch was comparable to the wavelength¹³ of green light, it was mostly green light that was reflected and therefore seen.

He and his colleagues demonstrated this effect with a dramatic display - taking photographs of the beetle under left and right circularly polarised light.

Under left circularly polarised light, which matched the direction of the helices, the beetles¹⁴ appeared "super-green", because, as Dr Srinivasarao explained, "there weren't that many other wavelengths¹⁵ of light reflected back at you".

"Under natural light, the maximum intensity¹⁶ of the green colour can only ever be 50%," he said.

Scientists already are studying ways to use materials that have properties similar to the beetles.

Researchers in New Zealand, for example, are studying beetles to produce a thin, solid mineral, magnesium¹⁷（镁） oxide¹⁸, which can be ground into flakes¹⁹ and potentially used as a currency security measure.

This finding reveals another natural template（模板，样板） that materials scientists can study and copy.