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Shenrich91 via Wikimedia Commons // CC BY-SA 3.0
Shenrich91 via Wikimedia Commons // CC BY-SA 3.0

Black Widows’ Hourglass Markings Deter Potential Predators

Shenrich91 via Wikimedia Commons // CC BY-SA 3.0
Shenrich91 via Wikimedia Commons // CC BY-SA 3.0

Everyone knows to watch their step around a black widow—including predatory birds. A new study shows that birds know to avoid the spiders based on their bright red markings. Meanwhile, the spiders' prey has a much more difficult time making out the markings, leaving them unaware of the lurking danger. The findings of the study were published last week in the journal Behavioral Ecology.

The species collectively known as black widows hold a special place in the human imagination. The pop mythology surrounding the female black widow’s potent venom and characteristic red hourglass markings have inspired songwriters, comic book creators, and even video game designers—a fact that the authors of the current study exploited. 

“Video game developers love to put black widows in their games because they’re scary,” lead author Nicholas Brandley said in a press release. Brandley and his colleagues bought a digitally rendered black widow (in this case, Latrodectus mactans) from a video game developer. They adapted the code for a 3D printer, and made black widow models in the lab. Four of the eight fake spiders were painted all black. The other four got the full killer-spider treatment in "Heavy Body Mars Black" and "Berry Red"—commercially available colors that closely match the coloration of live black widows. 

Actual spider belly. Image Credit: Nicholas Brandley

The scientists started by testing spider-eating birds. They set up seven bird feeders around Durham County, North Carolina, and replenished them daily with seeds so local birds would get used to visiting. Then, on test day, they took all the seeds away and replaced them with a single fake spider, laid belly up. 

They watched as the birds (Carolina chickadees, northern cardinals, tufted titmice, white breasted nuthatches, and two kinds of woodpeckers) approached the feeders. They found that the birds were 2.9 times more likely to attack the plain black spiders than the faux black widows. Smaller birds were especially likely to take off at the sight of the red hourglass. “The birds would see a spider model with red markings and get startled and jump back, like ‘Oh no man, get me out of here,’” Brandley said in the press release. 

Clearly, the birds could see the spiders’ markings and understood what they meant.

Next, the researchers measured the wavelengths of light given off by the markings of two black widow species. Bird eyes, human eyes, and insects all pick up different wavelengths, which affects how we all perceive color. They found that the black widows’ red markings are half as visible for insects as for birds. 

The scientists then wondered if viewing angle had anything to do with it, since black widows like to hang belly-up beneath their webs. This position would display the red markings to spider-eating birds flying overhead while conveniently concealing them from insects below. 

To find out, they set up 20-inch-tall “widow towers” in the lab, brought in two real black widow species (L. variolus and L. mactans), and let them set up house. Time and time again, L. variolus spiders, which also had red marks on their backs, built their webs higher in the tower than spiders from the other species, which did not. This suggests that the L. variolus spiders were raising their vivid markings even farther from insect view, making them harder to see. 

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8 Ways Spiders Are Creepily Clever
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You may already know that spiders can spin intricate webs and poison their prey. But that doesn't even begin to cover the all the sneaky abilities spiders have adapted to become the most fearsome organisms on eight legs. Here are some of the tricks spiders use to catch their meals while avoiding becoming dinner themselves.

1. THEY HAVE SUPER-POWERED SENSES.

Spidey-senses weren't just invented for comic books. Jumping spiders in real life have sharp eyesight and excellent hearing to make up for their inability to spin webs. Scientists long assumed that spiders couldn't hear because they don't have ears. But as researchers reported in a 2016 study, jumping spiders can "hear" perfectly fine—they just use the super sensitive hairs on their legs to do so. These same spiders can also see surprisingly well, as astronomer Jamie Lomax demonstrated when she used laser pointers to lure them away from her desk like they were tiny cats.

2. THEY MIMIC ANTS.

The fact that the jumping spider species Myrmarachne formicaria tricks predators into thinking it's an ant by mimicking its appearance isn't a new discovery. But exactly how it achieves this was unclear until recently. According to a Harvard study published in the Proceedings of the Royal Society B, the spider pulls off this deceptive stunt while using all eight legs to walk. During its performance, it takes 100-millisecond pauses to lift its front two limbs to its head so they resemble antennae. The switch is so fast that to a human looking from above, the spider appears to simply be walking with its back six legs while lifting its front legs off the ground. Scientists had to use high-speed cameras to prove this wasn't the case. 

3. THEY TUNE THEIR WEBS.

Despite lacking ears, spiders have some impressive musical talents. They treat the strands of their webs like the strings of a guitar, tuning them just right so they can detect certain vibrations. For their study published in the Journal of the Royal Society Interface, researchers from the University of Oxford and Charles III University of Madrid observed garden cross spiders maintaining their webs. They learned that adjusting the tension and stiffness of the silk allows the spiders to sense frequencies they can recognize. One signal might mean that prey is near, while another could be connected to structural issues with the web.

4. THEY PRETEND TO BE BIRD POOP.

Spider disguised as bird poop.

Min-Hui Liu et. al, Scientific Reports // CC BY-NC-ND 3.0

Camouflage is not unique among arachnids, but orb weaver spiders may win the prize for the most memorable disguise. In its juvenile stage of life, the spider will surround itself with a thick, white material in the center of its web. Its whitish abdomen blends into the "decoration," making the spider appear as if it's buried in a splatter of bird droppings. The unappetizing look is usually enough to convince predators to look elsewhere for a meal that's easier to stomach.

5. THEY CAST NETS.

Spider with web between it's legs.

Chen-Pan Liao, Wikimedia Commons // CC BY-SA 3.0

Bigger isn't always better when it comes to webs. Take the net-casting spider: The silken trap it uses to snare food is small enough to fit between its limbs. The spider poops out a pale "target" onto the forest floor and then hangs above it waiting, sometimes for hours, for an insect to come along and trigger a "trip wire" connected to the ground. Once that moment comes, it wastes no time lunging at its prey and enveloping it in its web. It then bites and paralyzes its prey before commencing the feast.

6. THEY CAN FIRE THEIR HAIRS LIKE TINY BARBED SPEARS.

If all else fails, at least tarantulas have their spear-like hairs to fall back on. A tarantula deploys its "urticating hairs" when it feels threatened. By grinding its back legs against its abdomen, it's able to shoot the barbed hairs at its target like a shower of tiny throwing stars. You don't have to be a predator to trigger this defense mechanism, as many tarantula pet owners have found out the hard way.

7. THEY SOMERSAULT.

When most spiders need to escape a dangerous situation, they rely on their eight limbs to scurry them to safety. The golden wheel spider curls up its body and rolls down hills to make an even speedier getaway. This type of spider is native to the Namib Desert in southern Africa, where steep, sandy dunes are abundant. When it's tucked into a ball, the spider can reach tumbling speeds of 3.2 feet per second.

8. THEY CREATE BUBBLE SUBMARINES AND SCUBA SUITS.

Even without gills, spiders have adapted some pretty clever ways of surviving underwater for long amounts of time. The diving bell spider weaves web balloons that extract dissolved oxygen from the water around it while filtering out carbon dioxide. Using this improvised scuba suit, the spider can last a whole day before it needs to come up for air. Then there are wolf spiders, which use a much more dramatic survival tactic. A 2009 study found that marsh-dwelling varieties of wolf spiders appear to drown after being submerged for extended periods. But once they're placed on dry land, they twitch back to life. Slipping into a coma underwater is how they're able to evade death.

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We May Be Hardwired to Fear Snakes and Spiders
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Just the mere sight of a daddy longlegs or garter snake can prompt shrieking and shoe-throwing, even though not everyone has had bad experiences with creepy-crawlers. Are we naturally predisposed to hate tiny critters that scurry and slither? A new study suggests so, according to a video from National Geographic (below).

The video highlights a new study by a team of researchers from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, and Uppsala University in Sweden, who measured the pupils of 6-month-old babies as they stared at pictures of flowers and fish, or snakes and spiders. Human pupils naturally enlarge as a response to danger—and sure enough, the babies' eyes dilated more frequently when they were exposed to the garden pests. This suggests that our widespread dislike of spiders and snakes might be ingrained in us (although the time your bunkmate hid a spider in your sleeping bag at camp probably didn't help, either).

You can check out the full study online in the open-access journal Frontiers in Psychology. And to help squelch your fears, here are some common (yet wildly inaccurate) myths about spiders, and some dispelled misconceptions about snakes.

[h/t National Geographic]

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