The U.S. biologists, found tarantula species have evolved almost exactly the same shade of vibrant blue at least eight separate times.
The hue is created by tiny structures inside the animals' hairs, but those shapes vary across the family tree.
The striking blue is not driven by sexual selection and tarantulas have poor colour vision. So, it is not a courtship show off.
Nonetheless, Bor-Kai Hsiung and his colleagues found that 40 out of 53 groupings (genera) of tarantula exhibit a very vibrant blue. "We collected published data and constructed a super-tree, which combined the previous published small trees," said Mr Hsiung, a PhD student at the University of Akron in Ohio and the first author of the study, published in Science Advances.
The blueness was mapped onto that evolutionary tree based on a bank of tarantula snaps scoured from the internet. "Eight is the lowest number, so it's [evolved] at least eight times."
All these blue spiders evolved almost within a tight 20 nanometre range, clustered around 450nm – a bright, cobalt blue. The team used electron microscopes to zoom in on tiny shapes within those hairs and conducted simulations to produce a "structural colour" by reflecting – very specifically – this wavelength of blue light.
"It evolved from multiple origins and different mechanisms produce the very same blue colour suggesting that this blue colour conceals the tarantulas from their prey while they hunt at night or warns enemies".
"We don't know yet," Mr Hsiung said.