Rainbow spot design challenges engineers

Dazzling colours of the Philippine snout weevil

by Jonathan Sarfati

phys.org

A number of creatures have striking iridescent colours. Normal pigments can’t generate them, because they simply absorb some colours and reflect others. Instead, the iridescent colours are structural, caused by photonic crystals, with a nanostructure that manipulates different colours differently. Photonic crystals are also responsible for the iridescent blues and extremely deep blacks seen in some butterflies. One butterfly even generates two iridescent signals, not just one.

Some other creatures can change colours, including iridescent ones. Some of the best known are the chameleon, cuttlefish, squid, and octopus. They have a number of different types of colour-producing cells (chromophores), including those that produce iridescence (iridophores).

Rainbow spots

An intriguing case of dazzling colour is the Philippine snout weevil (Pachyrrhynchus congestus pavonius). Its wing cases and thorax are covered with bright spots comprising concentric rings of most of the colours of the rainbow in the same order: from the outside, the colours are red, orange, yellow, green, cyan (turquoise), to blue in the centre.

Vinodkumar Saranathan of Yale-NUS College and Bodo Wilts from the University of Fribourg showed that the colours are caused by a network of chitin (the material in insect exoskeletons). This network has the same symmetry as the normal ‘cubic diamond’ crystal structure. This symmetry means that the colours don’t change regardless of the viewing angle, unlike many of the other photonic crystals in nature.

The particular colour depends on the size of the crystals and volume of chitin: red produced by the largest, and blue by the smallest.¹ According to a report on this work,² “this ability to simultaneously control both size and volume factors to fine-tune the colour produced has never before been shown in insects, and given its complexity, is quite remarkable.” Dr Saranathan further explained:

It is different from the usual strategy employed by nature to produce various different hues on the same animal, where the chitin structures are of fixed size and volume, and different colours are generated by orienting the structure at different angles, which reflects different wavelengths of light.

Biomimetics

Such structures could have great applications for digital displays or cosmetics with bright colours that could be viewed from any direction without distortion.² However, making them is “currently challenging”, so studying these rainbow spots “may provide fresh impetus for bioinspired and biomimetic multifunctional applications”.¹

It’s notable that if engineers do manage to make such applications, they will be rightly praised for the ingenuity. But they made only a copy. What does this say about the One who made the original?

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