Just what that force is, though, is still a mystery.

Instead, these new observations suggest some other evolutionary force is at work, Hsiung says. That means it’s unlikely the blue hue evolved because spiders prefer an eight-legged lover colored like the sea. That’s why scientists suspect sexual selection-or choosing mates based on courtship displays and other physical factors-is primarily responsible for flashy, structure-based colors.īut, Hsiung and his colleagues argue, that hypothesis falls apart when one considers the lowly tarantula-a creature that, despite having eight eyes, can’t see very well (birds and butterflies, on the other hand, are known to have excellent eyesight). It’s a system that’s easy to tweak, and can rapidly evolve. These light-reflecting arrays are also responsible for the shimmering colors of beetles, butterfly wings, and bird feathers the colors they reflect depend on the alignment and spacing between individual crystals. In some tarantulas, that wavelength happens to be the same shade of blue. The crystalline array acts like a mirror that reflects only a particular wavelength of incoming light. Unlike the pigments that make plant leaves green and flamingos pink, a tarantula’s blue color comes from precisely arranged nanocrystals. “We just don’t know what that function is yet.” “The blue color definitely has a major function, and it’s very specific why they need this color,” Hsiung says. In other words, scientists suspect there’s a good reason the spiders are blue. What’s more, by taking a close look at blue hairs from eight different tarantulas, the team found that the spiders don't all create the blue color in the same way, meaning the color isn’t simply hitching a ride with a different trait that offers the spiders an advantage. In fact, as Hsiung and his colleagues reported today in Science Advances, blue coloration probably evolved independently at least eight times in tarantulas, which are among the most ancient of spiders. Poecilotheria metallica, a critically endangered species, is covered in an intricate geometric blue pattern the burrow-building cobalt blue tarantula ( Haplopelma lividum) is a slightly lighter hue, while the Singapore blue ( Lampropelma violaceopes)-a large, aggressive tree-dweller-has eight dark blue legs arrayed around a golden body. Many tarantulas are quite colorful, and it’s not uncommon for the spiders to wear blue.

“We didn’t find the answer to that question,” says the University of Akron’s Bor-Kai Hsiung, spoiling the punchline. Recently, a team of researchers set out to solve one of nature’s little mysteries and determine why, exactly, some tarantulas are such a vibrant hue. Some are stunningly beautiful-and bright blue. Others defend themselves by launching streams of excrement. Some tarantulas are bigger than your face.

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