Next time a big, blue tarantula comes crawling over, just ask yourself, "Why is it blue?" At least that's what a group of researchers did, and they realized it's a tough question, National Geographic reports. The Verge reports there are at least 40 tarantula species that are colored blue; in this study, researchers analyzed eight. The species ranged from the critically endangered Poecilotheria metallica to the big, aggressive, Singapore-blue Lampropelma violaceopes, and the researchers found the coloration appeared to evolve separately eight times in tarantulas. Even more fascinating, the color's source isn't a pigment, but rather nanocrystals in the spiders' hair that reflect blue light—and the nanostructures aren't the same in every species. This suggests the trait "it is not related to a different trait such as an ability to repel water," per a press release.
"The blue color definitely has a major function, and it’s very specific why they need this color," says study co-author Bor-Kai Hsiung—and attracting a mate likely isn't it. Tarantulas' eight eyes don't see very well, so Hsiung and his colleagues tossed that theory. Their conclusion? The blue "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?" study co-author Todd Blackledge tells the Atlantic. "It really is a situation where we’ve thrown our hands up. I’m actually waving my hands as I’m talking to you." On the upside, "tarantula blue" isn't iridescent—meaning it doesn't change hue at different angles—so it could show us how to build better screens for phones and TVs, the Verge reports. (Cops recently responded to a domestic violence call—against a spider.)