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Scientists have figured out why some tarantulas are bright blue

The vivid color isn't just to make it look festive.

A Cobalt Blue Tarantula (Hapolpelma lividum).
Bastian Rast

Tarantulas are intimidating arachnids. From long hairy legs to a propensity for nocturnal escapades, they definitely rank high on the list of "spiders you wouldn't want to encounter in the middle of the night". But despite their reputation and creep factor, some tarantulas are more festive than you'd expect -- some, in fact, are bright blue and green.

In a new study published in Proceedings of the Royal Society B, scientists have discovered that Hapolpelma lividum tarantulas -- once thought to be colorblind -- are actually able to perceive a certain amount of color. This doesn't necessarily mean that a tarantula could enjoy a rainbow, but it does lend significant influence to the hypothesis that these tarantulas develop such vivid colors in order to attract mates.

Researchers from Yale-NUS College and Carnegie Mellon University have hypothesised that blue colors may be used to attract and communicate to potential mates, while the green color of some tarantulas may be for camouflage and concealment in arboreal, or tree-dwelling, species.

The team surveyed the bodily expression of opsins, or light-sensitive proteins, within the eyes of tarantulas in order to determine if they have a full range of color perception -- the likes of which are normally found in spiders like the Peacock spider (a notoriously brightly colored arachnid). 

The results indicate that while they may not have the full spectrum available to them, they can certainly perceive the blue color that Hapolpelma lividum have developed -- and, using comparative phylogenetic analyses, the scientists concluded they likely have been born blue for millions of years.

Saoirse Foley, co-lead scientist from CMU, emphasized that although these conclusions are data-based, there's still room for further confirmation. "While the precise function of blueness remains unclear, our results suggest that tarantulas may be able to see these blue displays, so mate choice is a likely potential explanation," she said. 

"We have set an impetus for future projects to include a behavioural element to fully explore these hypotheses and it is very exciting to consider how further studies will build upon our results."