A Nanoscale Light Trick Is the Key to Peacock Spiders’ Super-Black Spots

A new study shows how patches of bumpy microlenses capture 99.5 of the light that hits them

Peacock Spider 7 (Maratus speciosus)

Peacock spiders in the genus Maratus are definitely one of the cutest and most colorful arachnids on the planet. Their technicolor abdomens and slick dance moves have turned them into internet superstars. But until now, researchers were stumped by how the teeny-tiny spiders produced the deep, ultra-dark black spots on the their colorful tummies. It turns out, it’s all a trick of the light, as researchers report in a new study published in the journal Proceedings of the Royal Society B.

Carolyn Wilke at Science News reports that researchers have figured out how the other vibrant colors are produced on the various species of peacock spiders, which range between just 2 to 5 millimeters and live in Australia. Pigments contained in barbed scales create the vibrant reds, whites and yellows on the spiders, while tiny nanostructures embedded in scales that are unique to the spiders bend light to create iridescent blues and purples.

In the latest study, researchers analyzed the spiders’ black patches under a scanning electron microscope. The team—lead by Dakota McCoy, a Harvard graduate student studying evolutionary biology—compared two peacock spider species, Maratus speciosus and M. karrie, to one closely related all-black Cylistella spider species. The team found tiny bumps, or microlenses, in the peacock spider’s black patches while the Cylistella spider was smooth.

By running simulations of how these bumpy surface interact with light, Bob Yirka at Phys.org reports the team found that the curved bumps reflect light in random directions and away from the viewer, making the black patches appear darker. The microlenses also direct the light in such a way that it interacts longer with the black melanin pigment on the spiders abdomen, causing more light to be absorbed. All of that means less than 0.5 percent of the light that hits the black spots is reflected, creating the ultra-dark appearance. That is approaching the amount of light absorbed by Vantablack, a man-made substance created from carbon nanotubes, which captures 99.965 percent of the light that reaches it.

The deepness of the black has an impact on the surrounding bright colors, too, creating an “optical illusion that the colors are so bright … they’re practically glowing,” McCoy tells Wilke.

According to the paper, the microlenses in the peacock spider’s super-black hue are similar to features found in certain flower petals, some tropical shade plants, a species of starfishes’ arms, and in moth eyes. They are also comparable to a bird-of-paradise’s feathers that produce a similarly mesmerizing black, but the mechanisms are slightly different. Last year, McCoy found that specialized barbules on the birds’ feathers absorbed 99.5 percent of light, the same amount as the spiders.

These deep blacks may really make the colors on male birds and spiders pop during mating displays, but McCoy tells Yong that they could also be useful to humans for non-mating purposes. She says figuring out how to reproduce these nanostructures could help increase the efficiency of solar panels, improve military camouflage and could be used by astronomers to keep light pollution out of telescopes.

Get the latest stories in your inbox every weekday.