These “structural colors” don’t rely on any pigment. If you were to crush the butterfly’s wings, their blue would disappear. Conversely, if you left them alone, they would never fade with time.

Blackledge and Hsiung found that these almost all the blue tarantulas rely on structural colors too. In some species, the hairs contain clear layers like the morpho butterfly. In others, there’s more of a spongy filling. But in either case, the principle is the same: There are microscopic structures whose spacing favors the reflection of blue light.

The Gooty sapphire ornamental tarantula (Michael Kern)

But why blue?

It’s unlikely that they’re using the color to communicate with each other, since tarantulas have poor color vision, and terrible eyesight in general. It’s also very unlikely that natural selection has just shaped the spider hairs in ways that, by complete coincidence, make them blue. After all, different species produce their blue colors using hairs of very different shapes.

Here’s another clue: The blue tarantulas are not just clustered in some corner of the tarantula family tree. Instead, the team found that these spiders have evolved blue colors on at least eight separate occasions. And yet, they have all converged on much the same shades of blue—a range of wavelengths that’s much narrower than, say, blue birds or butterflies.

That’s especially weird because structural colors are very flexible. Animals that use pigments have to synthesize or find entirely new molecules if they want to change colors. But those that rely on structural colors just need to tweak the distance between microscopic layers, and voila—they’d get yellow or red. But they don’t. They’re all blue, and not just any blue, but that blue. Tarantula blue.

“For them to be this specific, it means that the blue color must itself have some function,” says Hsiung. “We definitely think it has to have some kind of visual function.”

“It might be a signal to predators, or maybe it’s a blue that’s not particularly bright in a rainforest environment… and it makes it harder to… track… the spider?” says Blackledge, his words dying as he speaks them. “It really is a situation where we’ve thrown our hands up. I’m actually waving my hands as I’m talking to you.”

Do the blue species share any similarities that might hint at a shared function? “Other than being tarantulas, no,” says Blackledge. Some are blue all over, and others have blue only on particular body parts. Some are constantly blue, and others are only blue as juveniles. Sometimes, just the males are blue, or just the females, or both sexes.

“My best suggestion is that it’s camouflage so they blend into dark, shady forest backgrounds,” says Linda Rayor from Cornell University, an expert on spiders. Tarantulas are generally active at night, and most of the blue ones spend their time in silken retreats, deep burrows, and shady forests.