Imagine that the world is shades of gray and a little blurry, almost as if your lousy peripheral vision has taken over. This fuzzy field of view extends so far that you can make out dim shapes and motion behind you as well; no need to turn your head. The one bright spot is an X-shaped splash of color that moves with your gaze. At the center of this splash, everything is crisp and clear — a small window of sharp, colorful detail in a gauzy grayscape.
Add some blades of grass the size of redwood trees, and you’ve got an inkling of what the world looks like through the eight eyes of a jumping spider. It might be a bit like watching a poorly focused black-and-white movie on a 3-D IMAX screen that wraps around the room, while you hold a spotlight shining high-definition color wherever you point it. In other words, it’s really, really strange, at least compared with our two-eyed human perspective.
Jumping spiders, which are the family Salticidae, are best known for their hilariously flamboyant mating dances, their large front eyes that make for adorable close-ups and their itty-bitty size — some of the more than 6,000 known species of jumping spiders are smaller than a sesame seed.
But scientists are discovering that there’s much more to these diminutive arachnids. Researchers are getting a sense of what it’s like to be another animal by doing innovative experiments to go deeper into these spiders’ lives, probing their ability to see, feel and taste.
“Part of why I study insects and spiders is this act of imagination that is required to really try to get into the completely alien world and mind and perceptual reality of these animals,” says visual ecologist Nathan Morehouse of the University of Cincinnati.
Eye of the spider
Unlike bees and flies, which have compound eyes that merge information from hundreds or thousands of lenses into a single, pixelated mosaic image, the jumping spider has camera-type eyes, similar to those of humans and most other vertebrates. Each of the spider’s eyes has a single lens that focuses light onto a retina.
The principal eyes — the big forward-facing ones that just beg us to anthropomorphize — have incredibly high resolution for creatures that are usually between 2 and 20 millimeters long. Their eyesight is sharper than any other spider’s and is the secret behind their ability to stalk and pounce on prey with impressive precision. Their sight is comparable to that of much larger animals like pigeons, cats and elephants. In fact, human visual acuity is only about five to 10 times better than a jumping spider’s.
“Given that you can fit a lot of spiders in one single human eyeball, that is pretty remarkable,” says animal behavior researcher Ximena Nelson, who studies jumping spiders at the University of Canterbury in Christchurch, New Zealand. “In terms of size-for-size, there’s just no comparison whatsoever to the type of spatial acuity that jumping spider eyes can achieve.”
But that sharp vision covers only a small portion of the spider’s field of view. Each of the two principal eyes sees a narrow, boomerang-shaped strip of the world. Together they form an “X” of high-resolution color vision. An adjacent pair of smaller, less sharp eyes scans a wide field of view in black and white, watching for things that need the attention of the big high-resolution eyes.
On the side of the spider’s head are two more pairs of lower-resolution eyes that any human parent would envy. They let the spider monitor what’s happening behind it, a nearly 360-degree view that’s a real advantage for a small animal that is both hunter and prey. Indeed, a jumping spider might consider our 210-degree field of view rather pitiful.
But in other ways, a jumping spider’s visual world is not so different from ours. The animal’s principal eyes and first set of secondary eyes together do basically the same job as our two eyes with our high-acuity central vision and low-resolution peripheral vision. Like the spider, we focus our attention on a relatively small area and largely ignore the rest until something catches our attention.
“I totally think of them as completely analogous,” Nelson says. “The jumping spider’s solution is exactly the solution we’ve adopted. They’ve just adopted it in a much smaller manner.”
Giving the side eye
Each of the jumping spider’s four pairs of eyes has a different job and behaves independently, but they all work together. This cooperation opens up intriguing research possibilities for behavioral ecologist Elizabeth Jakob of the University of Massachusetts Amherst. “I’m really interested in how the eyes collaborate,” she says.
Jakob uses an ophthalmoscope modified to create a spider eye tracker. With removable adhesive, she tethers a female Phidippus audax to the end of a small plastic stick. Then, she hangs the stick with its spider in front of the eye tracker, the spider perched on a little ball facing a video screen. Once the spider is in position, Jakob plays videos. As the spider watches, she records how the principal eyes react.
The tracker shines an infrared light at the retinas of the principal eyes while a video camera records the reflected, X-shaped field of view. The reflection is later superimposed on the video the spider viewed, revealing exactly what the spider’s principal eyes were focused on. For a human, watching the combined video is like peering through a portal into the spider’s visual world.
Jakob teases out the relationships between the eyes by showing various images to different eye pairs. In a study published April 15 in the Journal of Experimental Biology, Jakob and her colleagues tried to determine which kinds of objects seen by the secondary eyes would prompt a spider to swing the principal eyes over for a sharper look. This test probes more than just how the eyes work together; it gets at what’s important to a jumping spider.
“It’s just so interesting to see what captures their attention,” Jakob says. “To just get this little window into their mind.”
First, a silhouette of a cricket — an appealing meal for a jumping spider — appears on the screen. You can tell when the spider’s principal eyes have locked onto the cricket because the boomerangs start wiggling, rapidly scanning the silhouette while slightly twisting one way and then the other. The spider’s principal eyes can do this sort of visual gymnastics because the retinas are at the back of two long, independently movable tubes, each controlled by six muscles.
To find out what might draw the spider’s focus away from the cricket, Jakob adds other images to an area of the screen that is within the field of view of the secondary eyes. Any interest in a black oval? Nope. Maybe a black cross? Or another cricket? Not impressed. How about a black oval that is shrinking? Still no. What if the oval is getting bigger? Bingo: The boomerangs quickly flit over to the expanding oval to get a better look.
A jumping spider’s principal eyes can concentrate on preparing to pounce on dinner, while the other eyes notice and ignore any number of less relevant things. But if those secondary eyes spot something that’s getting bigger, well, that could be an approaching predator that requires immediate attention. It’s a nifty design — one that could make an easily distracted human jealous.
“We’re swimming in a sea of potential stimuli all the time,” Jakob says. One way to filter that information is to prioritize, focusing attention on some things over others. “This is certainly familiar to any human trying to focus on reading one thing.”
Spotlight on color
Among mammals, humans and many other primates have exceptional color vision. Most people can see three colors of light — red, blue and green — and all the various combinations of hues in between. Many other mammals typically see just some shades of blue and green light. Many spiders may also have a crude form of color vision, but for them it’s usually based on green and ultraviolet light, which extends their vision into the deep violet end of the spectrum beyond what humans can see, and covers the blue and purple hues in between.
But some jumping spiders see even more. While at the University of Pittsburgh, Morehouse led a team that discovered that certain species have a filter squashed between two layers of green-sensitive photo-receptors that allows these spiders to detect red light in a small area at the center of their principal eye field of view. This adds red, orange and yellow hues to their world, which, along with the UV, gives them an even wider rainbow of colors than humans.
Seeing red can be handy since it’s often used as a warning, in nature as well as in the human–engineered world of red lights and stop signs. For jumping spiders, the ability to see red may have evolved as a way to avoid toxic prey. But once this new world of color was available to the spiders, they put it to good use. “The evolution of color vision seems to be followed by the rapid use of the new colors that they can see in courtship,” Morehouse says.
Using Jakob’s eye tracker, Morehouse is testing female jumping spiders to see what interests them about the colorful, frenetic dances males use to woo them. He’s finding that suitors employ a combination of movement and color that seems specifically designed to capture and hold a female’s attention by playing to her various eyes. She can only see color within the boomerang–shaped view of her principal eyes, and the red, orange and yellow hues only at the center. Unless he can grab the attention of her secondary eyes with movement, she won’t turn her principal eyes toward him and may never see his fabulously colored features. And for him, this could be a matter of life and death, because an unimpressed female may decide to make a meal of him instead.
The males of one species Morehouse studies, Habronattus pyrrithrix, have a dazzling red face and beautiful lime-green front legs. Yet the females seem most impressed by the orange knees on the males’ third set of legs. When a male first spots a female, he raises his front legs like he’s directing a plane into its gate and skitters side to side, hoping to catch the attention of her secondary eyes. When she turns his way, he comes closer and starts flicking the wrist joints at the end of his raised front limbs. You can almost hear him saying, “Hey lady, over here!”
Once he’s drawn her attention, out come the orange knees. “They move them up behind their back into view in a kind of a peekaboo display,” Morehouse says.
To find out exactly what it is about a male’s display that turns a female’s head, Morehouse uses a bit of subterfuge. He doctors videos of males dancing, then plays the videos to a female perched in an eye tracker to see how each of the changes affects her attention. If the male has an orange knee hiked up, but it’s not moving, she’s less interested. If the knees are moving but the orange color is removed, she’ll look but quickly lose interest. He’s got to have both the right look and the right moves.
“He’s using motion to influence where she’s looking, and then he’s using color to hold her attention there,” Morehouse says.
Behavioral ecologist Lisa Taylor of the University of Florida in Gainesville, likens the males’ tactics to those of human advertisers. “It feels like a lot of the tricks that marketers use to influence our decisions,” Taylor says. “Understanding the psychology of spiders sometimes feels similar to understanding the psychology of humans.”
Can you feel it?
The leg-waving, knee-popping spectacle of the male courtship display is meant to capture female attention. But this dance is only one part of the show, behavioral ecologist Damian Elias of the University of California, Berkeley discovered.
Many spiders use vibrations to communicate, and there had been a few reports that jumping spiders were among them. When Elias investigated further, he found that male jumping spiders accompany their moves with a remarkably elaborate serenade of vibrations, sent through the ground to the females, beyond human perception.
“It was a complete surprise to me,” Elias says. When he shared what he had found with other arachnologists, “they were just blown away.”
To eavesdrop on the spiders’ seismic songs, Elias uses a laser vibrometer, similar to technology used in the aeronautics industry to measure vibration of airplane components. He tethers a female spider onto a nylon surface stretched like a drumhead and then adds a male. When the male spots the female, he starts his song and dance, drumming his legs on the surface and vibrating his abdomen.
The vibrometer measures the vibration of the surface and translates it into airborne sound that humans can hear, revealing an acoustic barrage of thumps, scrapes and buzzes. Elias simultaneously records video of the courtship at 1,000 frames per second so he can slow it down to see how the sound and motion sync up. It’s like a miniature drum solo, perfectly matched to the spider’s flicks and kicks.
“We don’t have access to that without technology,” Elias says. “It’s kind of unlocking this secret world.”
The jumping spider’s sensory world is filled with vibrations coming through the ground. But because those vibrations feel different depending on what the spider is standing on, things can change quickly as he hops from leaf to rock to soil.
For humans, who sense sound vibrations through the air rather than the ground, Elias imagines it might be something like taking two steps and suddenly listening through water, and then another two steps and you’re surrounded by foam, and then back to air. The spiders’ entire sensory world is constantly changing, yet they adapt without missing a beat.
Now, take that alien auditory world and add the fact that jumping spiders also have chemical sensors at the tips of their legs. “They’re moving around, tasting everything that they’re walking on,” Elias says.
Very little is known about this aspect of the jumping spider’s sensorium, but the latest research out of Taylor’s Florida lab, published July 29 in the Journal of Arachnology, suggests that male spiders may be hoping to taste traces of potential mates. Most jumping spiders don’t build webs to capture prey; they stalk and pounce instead. But the spiders are constantly laying down a line of silk as they move about, a sort of safety rope in case they fall or need to make a quick escape.
In the new study, a male H. pyrrithrix could sense a female’s silk line when he stepped on it. Taylor’s lab is now testing whether a male spider can detect the difference between a silk trail that will lead him to a virgin female who might be willing to mate with him and a dragline left by a female who has already mated and might prefer to eat him.
“The more we learn, the more complicated it gets,” Taylor says. Jumping spiders “are so highly visual, and there’s so much vibrational stuff going on, and then the chemistry. It’s hard to imagine that it wouldn’t just be super overwhelming.”
Somehow, jumping spiders manage the sensory deluge quite well — they live just about everywhere on Earth. You’ve most likely seen one, possibly in your own house. Despite being so small, they are easy to identify if you know what you’re looking for — or what they’re looking for.
“Next time you see a spider in the middle of a wall, and you look at it, and it turns back and looks at you, that’s a jumping spider,” University of Canterbury’s Nelson says. “It’s detected your movement towards it with its secondary eyes, and it’s checking you out.”
That spider might just be imagining what the world looks like to a human.