How peacock spiders use optical illusions to woo females

Tiny arachnids known as peacock spiders are remarkable for many reasons: their adorable many-eyed faces, elaborate courtship performances, and ninja-like jumping skills.

Males also sport extremely colorful rear-ends, and will shake a florid array of blues, purples, golds and reds in a quest to woo potential mates.

Now, scientists report, these fingertip-size spiders may also rely on super black patches, interspersed between their technicolor hues, to dazzle the ladies. (Read about the wild sex lives of spiders.)

“If you frame a bright color in super black, it looks awfully different,” says Dakota McCoy, a graduate student at Harvard University who recently described these intensely black patches in two jumping spider species in the journal Proceedings of the Royal Society B.

“We think the black makes them look more beautiful by emphasizing nearby colors.”

According to the team’s observations, spider super black reflects less than 0.5 percent of incoming light, creating a gleam-free, featureless void in color.

That means these spider butts are among the most light absorbent in nature, on par with birds of paradise, another group of species in which males vie for females using flashy, colorful performances.

So in case you’re wondering how much more black those patches could be, the answer is almost none—almost none more black.

Optical illusions

Recently, McCoy and her colleagues studied a similar type of super black coloration in male birds of paradise. They found that their blackness is produced by microscopic structures that imprison the majority of incoming light, creating a featureless, dark surface.

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After watching videos of various peacock spiders performing their eight-legged choreography, McCoy wondered whether a similar mechanism might be crafting the dark, decidedly unflashy patches punctuating the arachnids’ colors.

“We noticed they also had dark black near bright color and thought to ourselves, How wild would it be if these distantly related animals, with similar lifestyles, evolved exactly the same visual illusion?” McCoy says. (See pictures of peacock spiders nicknamed Sparklemuffin and Skeletorus.)

So she and her colleagues took a close look at super black patches in Maratus speciosus and Maratus karrie, native to Australia. An electron microscope revealed that in both species, the patches contained bits of melanin—a black pigment—tucked beneath a layer of bumps in the spider cuticles.

Based on comparisons with smooth, less-black spider butts, plus simulations of how light interacts with the bumps, the team says the bumps likely act as microlenses. By trapping and focusing light onto the dark pigment, the microlenses eliminate reflections and make the patches appear more black.

In addition, M. karrie had a layer of fine, brush-like scales parked atop the bumpy lenses, which the team suspects are functionally similar to barbules in bird feathers.

Together, the pigment and associated light-manipulating structures create super black patches, which—when positioned next to colors—trick female spiders into perceiving the surrounding colors as almost impossibly bright.

“This is pretty amazing if true,” McCoy says, noting that spider visual systems are quite different from other animals. But, “maybe super black reveals a very fundamental sensory bias—a fundamental requirement of color vision that also generates optical illusions.”

Smart spiders

It’s not surprising to find that peacock spiders are using super black patches to enhance surrounding color, says Bill Hsiung, a postdoctoral scholar of the Scripps Institution of Oceanography who has extensively studied spider coloration.

In fact, peacock spiders employ a complex variety of techniques to create their back-end bling, with their flickering reds and golds based on pigments, and the iridescent blues and purples being the work of light-reflecting structures.

Hsiung agrees that a combination of structure and pigment creates the super black patches in peacock spiders. But he isn’t quite sure the structural component is nailed down yet.

“Understanding how color is (not) produced is a step toward deciphering the biological function of such color,” Hsiung writes in an email.

“We might never know for sure how these tiny spiders perceive things, [but] research like this shows that their tiny brains might be more capable than we gave them credit for.”

Spiders of paradise

Super black occurs in many other species—deep-sea fish and butterflies likely use it for camouflage, while snakes may have black scales to signal toxicity. So far, birds and paradise and peacock spiders are the only animals thought to use it to attract mates.

Still, to really prove that super black affects the mating choices of female peacock spiders, McCoy says additional behavioral experiments are needed. (Learn how tough it is to impress female peacock spiders.)

“Peacock spiders and birds of paradise are two of the most elaborate types of animals on Earth,” McCoy says.

“I sometimes call them 'spiders of paradise'—but of course, that’s the bird-centric view of the world.”