It’s a system that’s easy to tweak, and can rapidly evolve. 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.

But, 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). That means it’s unlikely the blue hue evolved because spiders prefer an eight-legged lover colored like the sea.

Instead, these new observations suggest some other evolutionary force is at work, Hsiung says. Just what that force is, though, is still a mystery. Hsiung suspects the color is some kind of signal, but it’s not clear who it’s for or whether the message is “come here” or “don’t eat me.”

But Vinod Saranathan, who studies the physics and evolution of structural colors at Yale-NUS College in Singapore, isn’t quite ready to accept that sexual selection hasn’t played a role in creating these blue-hued beauties, noting that in some tarantula species, males and females are colored differently. Perhaps, he says, mate preferences drove the development of colors early on, and the colors no longer offer the spiders any particular advantages.  

Hsiung says that while some male and female tarantulas do differ in color, the placement of the blue hue—and the fact that it’s not always present in adults of reproductive age—isn’t always what you’d expect for a trait that helps spiders make more spiders.