We Now Know How Peacock Spiders Produce Their One-Of-A-Kind Rainbows

Look at M. robinsoni's shiny spider butt. Jurgen Otto/Flickr; CC-BY-NC-ND 2.0

If you haven’t heard of peacock spiders by now, then where have you been? Have your eyes not been graced by the remarkable sight of these most vibrant of beings, gyrating and gesticulating back and forth, hoping to attract a mate?

Far from just being nature’s most adorable purveyors of flamboyancy, a recent revelation reveals that their ability to produce a rainbow of colors may help our own species make advances in the field of optics.

As reported in a new Nature Communications study, an international team of researchers has been closely examining two miniature Australian peacock spiders: the rainbow peacock spider Maratus robinsoni, which they argue is the most impressive of the bunch, and Maratus chrysomelas, another indubitable showoff. They’ve zeroed in not on their behavior, but their colorful displays themselves.

When you think of colors, pigmentation – paint, say – normally comes to mind. That’s one way of generating color, but it’s not the only way.

Plenty of animals, including these two peacock spiders, use something called structural coloration. Generally speaking, this involves multiple layers made of very specific shapes, which alter the direction and speed at which light enters them.

Light falls on the top layer first, and some of it is reflected; some light, however, penetrates down to the lower layer, where it is reflected there. Both reflected waves make their way back up to the surface, but by this stage, they’re traveling across different distances and perhaps at different angles.

That means that when they meet up again at the surface, they will either interfere with each other constructively or destructively – "add" or "subtract" – which ultimately produces a range of colors. This process is known as structural coloration, and involves a range of incredibly small structures and materials. Incidentally, the gradual changing of color depending on the angle of viewing is known as iridescence, something you can readily observe in a soap bubble.

The team behind this study, led by the University of California San Diego, wanted to know how these two peacock spiders produced their own characteristic iridescence, something that no other beastie is able to replicate.

“These two species of peacock spiders raise and wiggle their abdomens toward potential mates during courtship to display every color across the entire visible spectrum” – a complete rainbow of color – “making this the first true rainbow-iridescent signal known in animals.”

A range of techniques were employed to find out how they do this, including using electron microscopy, which uses incredibly precise beams of electrons to uncover the finest details of a sample. When they began to spot a few telltale structures, they opted to use 3D printing to produce nano-scale prototypes in order to test out their ideas.

It turns out that these spiders are armed with novel scales that coat their abdomens. As well as being curved in highly specific ways, the scales also appear to be adorned with a grate-like pattern, one that diffracts (bends) waves of light.

Combined, both features allow these critters to manufacture a full spectrum of color. Considering how spectacular the visual senses of the peacock spiders are, the team suspect that this ability likely evolved as “a direct product of sexual selection through female choice.”

As cool as this discovery is in isolation, the team highlights how this could help engineers develop remarkable new materials. This ability to produce a rainbow of color on such a small scale and to such a remarkable degree of precision could lead to plenty of bio-inspired designs – anything from fancy advertisements to highly portable spectrometers.

These little legends truly are the gift that keeps on giving.

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