Blue spider scales aren’t what they seem

Maratus splendens (photo J. Otto/Flickr)

Tiny Australian male peacock spiders of the genus Maratus, members of the jumping spider family, have become renowned over the past few years for their exquisite colors thanks to Sydney-based photographer Dr. Jürgen C. Otto. One of them, the Maratus splendens, has arguably one of the more admirable colorations, due to a rich pattern featuring white, blue, cream and red scales covering its body.

The striking pattern allows the male spider, who is only three to five millimeters long and totally harmless unlike some of his Australian counterparts, to charm attentive females. But where does he get his vibrant colors from?

Researchers from the NCCR Bio-Inspired Materials at the Adolphe Merkle Institute (University of Fribourg), the University of Groningen and Otto have published a new study in the Journal of the Royal Society Interface on how the spiders display these various hues.

They characterized the optical properties of the spider scales in detail by using advanced optical and anatomical techniques. They were able to show various scales derive their color from different pigments (chemicals) and scattering spines, except for the blue scales.

These blue scales are in fact unpigmented (or transparent) and have a structural color, created by an intricate, complex photonic system, which consists of two layers of chitin, the base material of insects and spiders, separated by an air gap.  

The structure of the scales was determined first by a scanning electron microscope and the picture was then completed by optical modelling. This modelling analyzes the effects of how light interacts with these structures.

An array of filaments exists on the inner sides of the chitin layers, somewhat resembling the fiber arrangement in reinforced concrete. After modelling the structure, the researchers were able to show that the filament array constitutes a novel structural coloration system, which subtly fine-tunes the reflectance that results in the stunning blue coloration. SEM image of a blue scale (B. Wilts/Adolphe Merkle Institute)

“Structural colours in nature can be found in many organisms,” says Adolphe Merkle Institute senior scientist Bodo Wilts.

“The power of structural coloration is that small changes in the dimensions of the nanosized structures cause strong color changes, which are not readily realized with pigments. Evolution has brought forward a diversity of mechanisms to create structural color; however, the peacock spider structure seems to be uniquely evolved.”

According to the researchers, the spider seems to have optimised the amount of material to reflect blue light, something they can definitely learn from. Other animal nanostructures working on the same principle of structural color are being considered for projects such as improved television screens or more efficient solar panels.

“Jumping spiders are beautiful visual animals where the coloration mechanisms have hardly been studied at all. Investigating the variety of structures in the family will definitely be interesting,” adds Wilts.

The researchers believe the peacock spider can actually detect the full range of colors, although how it achieves this is still the subject of ongoing studies.

Reference: Stavenga DG, Otto JC, Wilts BD. 2016 Splendid coloration of the peacock spider Maratus splendens. J. R. Soc. Interface 20160437

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