When tarantulas grow blue hair

Roses are red. These tarantulas are blue. They can’t see their color, so what does blue do?

Among the various tarantula hues, “the blue color is mysteriously widespread, ” says Bor-Kai Hsiung of the University of Akron in Ohio. At least one species of tarantula flashes some blue in 40 of the 53 genera he checked. Mapping them onto a tarantula family tree, Hsiung estimates that the color evolved independently at least eight times. At first, he wondered if color among the colorblind spiders was a by-product of some more useful trait. But evidence led elsewhere.

Tarantula blue comes solely from hair. Underneath even the most eye-popping sapphire locks, spider cuticle is a dull dark. Like 90 percent of the blue coloration in nature, Hsiung says, the tarantula hair comes to its blueness from light reflected through embedded nanoscale structures. And, oh the variety. Some species deploy an ordered phalanx of micro pancake stacks, each with cuticle layers alternating with thin air pockets. And for the first time in spiders, Hsiung also found “very, very subtle” arrays (called quasi-ordered spongy structures) like those seen in some blue feathers.

The structures and evolutionary histories may differ, but the spiders are “evolving the same blue over and over again,” says coauthor Todd Blackledge, also at Akron. Tarantula blue, surely a paint name for the future, lies around the 450-nanometer wavelength, give or take 10 nanometers — a pretty narrow range, Hsiung and his colleagues report in the November 27 Science Advances. “Between a navy blue and sky blue,” Hsiung says.

With the convergence on a narrow range, the blue seems more than an accident. Maybe in the green-tinted light of forests, a blue tarantula would be hard for predators (or prey) to see, Blackledge speculates. Hsiung wonders if the blue looks enough like the coloring of some fierce wasps to make hungry predators hesitate before attacking. Or maybe the blue resembles flower petals enough to lure some insects within spider-striking distance. All the researchers can say with some confidence is that tarantula blue has little to do with romance. Or roses.