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article imageCute, colorful jumping spiders can see more colors than humans do

By Megan Hamilton     May 20, 2015 in Science
Sometimes surprises come in tiny packages. That's the case with jumping spiders, scientists have found.
Scientists know that jumpers have excellent vision. What they didn't know until now is that these little creatures can see more colors than we do.
Researchers have discovered that these enchanting little spiders see in three color "channels," just like we do, according to Science World Report.
"The eyes of jumping spiders could not be more different from those of butterflies or birds, and yet all three tune the color sensitivities using pigments that filter light," researcher Nathan Morehouse said in a news release. "It's actually a pretty clever, simple solution with a big payoff."
Jumping spiders come in an array of flamboyant colors — including fire engine red and brilliant green — and these colors are meant to wow the ladies — female jumping spiders, that is. Yet it's long been a mystery as to how their eyes perceive color, National Geographic reports.
Scientists were aware that some jumping spiders, particularly Habronattus pyrrithrix, could see green and ultraviolet (UV) light through two of their eyes. Jumpers, it should be noted, have eight eyes. The researchers assumed the spiders could see oranges and reds — why else would the males have patches of these flashy colors?
So, the researchers conducted experiments with H. pyrrithrix, and it wasn't long before they had the answers. These little jumping spiders use filters. Sitting in front of the cells in their eyes, red filters detect green light, scientists reported online in the journal Current Biology on May 18. This produces an effect similar to that of gels that are placed in front of theater lights to show different colors on a stage. However, the spiders can't remove their red filters because they are made of a type of pigment.
These built-in filters give H. pyrrithrix a third color channel to work with.
"In principle, they can see an even broader spectrum of colors than we can," says evolutionary biologist and study co-author Nate Morehouse, of the University of Pittsburgh in Pennsylvania. This is because the spider is "sensitive not only to the spectrum of colors visible to us, but also to the UV."
In most cases, an animal this tiny has to make trade-offs between sight and the spatial resolution and sensitivity of their eyes, says sensory ecologist Daniel Zurek, also of the University of Pittsburgh, and the study's lead author.
While everything we need to enjoy the sight we have — like specialized cells that pick up two or more colors — won't fit in spider-sized eyes.
This hasn't crimped the jumping spider's style however. They circumvent the trade-offs by breaking up the duties that one pair of eyes performs in people and distribute them among four pair of eyes, National Geographic reports.
"Spiders have eight eyes that are specialized," Morehouse says. Some of their eyes detect motion or garner wide-angle views of their surroundings.
Zurek and Morehouse focused on H. pyrrithrix's central pair of eyes. These are adapted for seeing patterns and color. The researchers used multiple techniques to study the eyes, including slicing them into thin sections and examining the slices with specialized microscopes to discover the filters.
Those filters, Zurek says, were completely unexpected.
"It's a sweet way of solving the problem," Gil Menda, a neuroethologist at Cornell University in Ithaca, New York, told National Geographic. He wasn't involved in the study.
Menda notes this isn't unknown to science. Bird and reptiles have their own filters to see colors, and these are made from oil droplets. Butterflies also utilize a filter system.
Zurek and Morehouse believe this is the first time filters have been detected in spider eyes.
"Trichromatic vision resulting from this filter system should markedly enrich these animals'perception of color, including reds, oranges and yellows often found in their courtship displays," the study authors wrote, noting that males often sport bright red faces, The Los Angeles Times reports.
What's particularly fascinating is that this upgraded color system only exists in a tiny portion of the spider's retina, which means they have a very narrow field of color vision. This could be problematic for a female jumping spider who's searching for Mr. Right, the scientists say. It may, however, be one way to explain their strange gaze movements, which a 1969 paper published in the Journal of Experimental Biology described as an "elaborate and unique 'scanning' process in which both retinae move back and forth across the target, while at the same time partially rotating about the visual axes of the eyes."
Spiders in the genus Habronattus and Maratus both appear to possess this unique three channel vision. These colorful spiders are in the family Salticidae, the Los Angeles Times reports. Spiders in this family are known to be tiny, but what they lack in size, they more than make up for in predatory ferocity, carefully stalking, then pouncing on their prey. Most interestingly, the position of the eyes on their heads gives them a 360-degree view. The genus Maratus, also known as the peacock spider, is found in Australia, while Habronattus is found in Northern and Central America.
This study will likely open all kinds of new questions for scientists, Menda notes, in National Geographic. He researches how processes in the brain produce different behaviors in animals. Now that researchers are aware that certain jumping spiders can see red, it could be interesting to see how the spider brain processes this information.
"We're really at the beginning of explaining" color vision in these spiders, Zurek says.
One topic for future study is whether it's possible that being able to see a wide range of colors may have helped the spiders to notice prey with the kind of red and yellow coloring that is usually meant to warn predators away from a potentially toxic meal.
It's very clear that this study has opened the door for a lot of new questions about how jumping spiders see their world.
Note: This video of the peacock spider clearly shows how the little creature earned its name.
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