How Peacock Spiders Make Rainbows on Their Backsides

The adorable arachnids use specialized scales to break light into its component colors to produce some of nature’s tiniest rainbows

Peacock Spider
A male peacock spider, Maratus robinsoni B.-K. Hsiung et al./Nat. Commun./CC BY 4.0

Peacock spiders may be the world’s cutest arachnids—and the only who are verified Youtube stars. Hailing from Western Australia, the creatures are known for their elaborate mating dances where they wave their legs overhead, shake their rears and hop around. But part of what is so eye catching about these minute creatures, who are just five millimeters long, are their thoraxes, which are covered with iridescent rainbows.

Those beautiful rainbow colors are the only display in nature that uses all the colors of the rainbow, reports Brandon Specktor at LiveScience, and researchers have now figured out just how the little spider produces the sparkly spectacle.

Two particular species of peacock spider, Maratus robinsoni (also known as the rainbow peacock spider) and Maratus chrysomelas, have particularly notable displays, according to a press release. To figure out just how the spiders produce their incredible shimmer, a team of biologists, physicists and engineers came together to study the scales on the spider’s thorax that produce the impressive color. Using techniques like electron and light microscopy, imaging scatterometry and optical modeling, the team created micro-3D models of the scales to test how they worked.

What they found is that the rainbow color is produced by a specialized scale shaped like an airfoil or airplane wing. According to Nature Research Highlights, parallel ridges on top of the scale act as tiny diffraction grating, able to divide visible light into its component colors. A slight curvature of the scale allows light to pass over more ridges, separating the light into the colors of the rainbow even more effectively than if the scales were flat. The research appears in the journal Nature Communications.

While it's interesting to find out just how the spiders create their sparkly masterpieces, it is also giving material scientists and engineers ideas for new ways to create such bright iridescent colors. “As an engineer, what I found fascinating about these spider structural colors is how these long-evolved, complex structures can still outperform human engineering," Radwanul Hasan Siddique, a postdoc at Caltech and a co-author says in the press release. “I wonder how the spiders assemble these fancy structural patterns in the first place.”

This isn’t the first time lead author Bor-Kai Hsiung has investigated smartly colored insects. In 2015, as a grad student at the University of Akron, he investigated the surprisingly numerous species of blue tarantulas in the world. As Ed Yong at The Atlantic reported at the time, Hsiung and his co-authors discovered that the blue colors in the tarantulas was also structural—created by the manipulation of light instead of being produced by a pigment or coloring, much like the peacock spider rainbows.

But the blue coloring of the tarantulas is not iridescent or shiny. Instead, it is a muted blue that might allow the arachnids to blend into shadows on the forest floor. This, too, is a useful property, Hsiung told Yong in 2015. By learning how the tarantulas produce matte colors, scientists could perhaps learn how to create long-lasting bright colors that don’t cause headaches.

“We usually don’t want colors to change over different viewing angles; it’s good eye candy but you don’t want to be living in a room with iridescent paint,” he said. “If we can mimic tarantulas and produce structural colors that are bright and non-fading, it might be useful for color displays on electronics, e-readers, TVs, or computers.”

The way the peacock spiders use structural colors to produce a rainbow also has lots of potential industrial applications. According to the press release, it could help make small optical spectrometers for space missions or could help produce wearable chemical detection systems.

But it might also actually end up on your living room walls. According to Katie Byrd at the Akron Beacon Journal, Hsiung’s studies on the tarantulas and peacock spiders was partially sponsored by the paint company Sherwin-Williams, but its unlikely the colors will hit the shelves as “Tarantula Blue” or "Rainbow Spider Thorax."

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