Why Don't Spiders Get Stuck in Their Webs?

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When a bug flies into a spider web, the game is over. It’s almost instantly stuck, and a sitting duck for the web’s owner. When you or I walk into a web, we’re a little better off than the bug because we won’t be dinner, but the sticky strands of web are still a pain in the butt to pick off of clothes and skin.

The spider itself, which spends much more time in contact with the web than you or any bug, doesn’t seem to have any issues getting stuck as it moves around. What gives?

For a long time, people thought spiders didn’t get stuck because their legs were coated in an oil made inside their bodies. With their legs lubed up like this, there was nothing for the silk web strands to stick to. Early 20th century naturalists proposed this idea — that the spider “varnishes herself with a special sweat,” as one elegantly put it — after observing spiders in the wild. The hitch is that, for all the research on spiders scientists have done in the meantime, no one had bothered to test the idea until recently.

A study published last year by two biologists in Costa Rica, Daniel Briceño and William Eberhard, suggests that spiders stay unstuck thanks to a combination of behavior, anatomy and, yes, even an oily non-stick coating.

What a Web They Weave

The first thing that helps spiders from getting trapped is that not every part of every web is sticky. In many orb weaver spider webs, for example, only the spiral threads are made with sticky silk. The “spokes” that support the structure of the web and the center part of the web where the spider rests are made with “dry” silk.

Using the center area and the spokes, a spider can move all around the web, and even off of it, without any concern for getting stuck.

Neat Feet

The spiders that Briceño and Eberhard studied used the dry threads for moving around most of the time, but when prey landed on the webs and the spiders went to retrieve their dinner, they inevitably had to charge across a sticky section. Unlike their prey, though, the spiders didn’t just whack into the sticky threads willy-nilly. The scientists found that the spiders walk very carefully when on the sticky sections, holding their body clear of the web and making minimal contact with the threads with only the tips of their legs.

Under a microscope, Briceño and Eberhard saw that the sticky threads do indeed make contact with the spider and stick to the setae, or short bristly hairs, on their legs. As a spider pulls its leg of the web, though, the droplets of adhesives that sit on the thread slide toward the edge of the bristle, where they have contact with only the thin tip and easily pull away. All these bristles are also in irregular rows and break free from the sticky droplets one by one, not all at once, which keeps the adhesive force of multiple droplets from combining.

Smooth Like That

What is it about the setae that lets them shed the web’s adhesives so easily? When Briceño and Eberhard washed a detached spider leg and applied it to a sticky thread, the leg stuck and wasn’t as easily removed. They figured that the bristles must have either a chemical coating of anti-adhesive substances or a structural surface layer with anti-adhesive properties. After analyzing several compounds washed off the the spiders’ legs, they found several several oily substances — including n-dodecane, n-tridecane, and n-tetradecane — that could act as a non-stick coating.

The researchers couldn’t tell where the chemicals had come from, but scientists’ descriptions from the last century suggested that they were applied by the spider’s mouth. Sure enough, when Briceño and Eberhard washed a live spider’s legs, it passed each of the legs through its mouthparts, but they didn’t test whether or not any anti-adhesive material was being applied.

To see if the spiders were coating their own legs would require a pretty simple experiment, Eberhard told me via email, but the spider they were working with, Nephila clavipes, is only seasonally abundant. The study would have to wait until the population climbed again, so the source of the non-stick chemicals is still a mystery for now. In the meantime, he said, he’s looking into how spiders deal with a different type of silk, called cribellum silk, which can be sticky without being wet.

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What Happens When You Flush an Airplane Toilet?
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For millions of people, summer means an opportunity to hop on a plane and experience new and exciting sights, cultures, and food. It also means getting packed into a giant commercial aircraft and then wondering if you can make it to your next layover without submitting to the anxiety of using the onboard bathroom.

Roughly the size of an apartment pantry, these narrow facilities barely accommodate your outstretched knees; turbulence can make expelling waste a harrowing nightmare. Once you’ve successfully managed to complete the task and flush, what happens next?

Unlike our home toilets, planes can’t rely on water tanks to create passive suction to draw waste from the bowl. In addition to the expense of hauling hundreds of gallons of water, it’s impractical to leave standing water in an environment that shakes its contents like a snow globe. Originally, planes used an electronic pump system that moved waste along with a deodorizing liquid called Anotec. That method worked, but carrying the Anotec was undesirable for the same reasons as storing water: It raised fuel costs and added weight to the aircraft that could have been allocated for passengers. (Not surprisingly, airlines prefer to transport paying customers over blobs of poop.)

Beginning in the 1980s, planes used a pneumatic vacuum to suck liquids and solids down and away from the fixture. Once you hit the flush button, a valve at the bottom of the toilet opens, allowing the vacuum to siphon the contents out. (A nonstick coating similar to Teflon reduces the odds of any residue.) It travels to a storage tank near the back of the plane at high speeds, ready for ground crews to drain it once the airplane lands. The tank is then flushed out using a disinfectant.

If you’re also curious about timing your bathroom visit to avoid people waiting in line while you void, flight attendants say the best time to go is right after the captain turns off the seat belt sign and before drink service begins.

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Why is Friday the 13th Considered Unlucky?
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Today, people around the globe will feel uneasy about getting out of bed, leaving their homes, or going about their normal daily routines, all because of a superstition. These unfortunate folks suffer from paraskavedekatriaphobia, a common neurosis familiar to us all: the fear of Friday the 13th. But just where did this superstitious association come from, and how did it catch on?

The truth is that no one is absolutely sure where the idea that Friday the 13th is unlucky originated. Donald Dossey, the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina, suspects the fear can be traced back to a Norse myth about 12 gods who had a dinner at Valhalla—the fabled hall where legendary Norse heroes feasted for eternity after they died—that was interrupted by a 13th guest, the evil and mischievous god Loki.

According to legend, Loki tricked Höðr (the blind god of winter and son of Odin, the supreme god in Norse mythology) into shooting his brother Baldr (the benevolent god of summer who was also a son of Odin) with a magical spear tipped with mistletoe—the only substance that could defeat him. Thus the number 13 was branded as unlucky because of the ominous period of mourning following the loss of such powerful gods by this unwanted 13th guest.

For whatever reason, among many cultures, the number 12 emerged throughout history as a "complete" number: There are 12 months in a year, 12 signs of the zodiac, 12 Gods of Olympus, 12 sons of Odin, 12 labors of Hercules, 12 Jyotirlingas or Hindu shrines where Shiva is worshipped, 12 successors of Muhammad in Shia Islam, and 12 tribes of Israel. In Christianity, Jesus was betrayed by one of his 12 Apostles—Judas—who was the 13th guest to arrive for the Last Supper. Surpassing the number 12 ostensibly unbalances the ideal nature of things; because it is seen as irregular and disrespectful of a sense of perfection, the number 13 bears the stigma of misfortune and bad luck we know today.

WHY FRIDAY?

Friday joins in the mix mostly because all of the early accounts of Jesus’s crucifixion agree that it took place on Friday—the standard day for crucifixions in Rome. As Chaucer noted in The Canterbury Tales, "And on a Friday fell all this mischance." Yet perpetuating Friday as an unlucky day in America came from the late 19th-century American tradition of holding all executions on Fridays; Friday the 13th became the unluckiest of days simply because it combined two distinct superstitions into one. According to the Oxford University Press Dictionary of Superstitions, the first reference to Friday the 13th itself wasn’t until 1913. (So despite actually occurring on Friday, October 13, 1307, the popular notion that the Friday the 13th stigma comes from the date on which the famed order of the Knights Templar were wiped out by King Philip of France is just a coincidence.)

The repercussions of these phobias reverberated through American culture, particularly in the 20th century. Most skyscrapers and hotels lack a 13th floor, which specifically comes from the tendency in the early 1900s for buildings in New York City to omit the unlucky number (though the Empire State Building has a 13th floor). Some street addresses also skip from 12 to 14, while airports may skip the 13th gate. Allegedly, the popular Friday the 13th films were so-named just to cash in on this menacing date recognition, not because the filmmakers actually believed the date to be unlucky.

So, is Friday the 13th actually unlucky? Despite centuries of superstitious behavior, it largely seems like psychological mumbo jumbo. One 1993 study seemed to reveal that, statistically speaking, Friday the 13th is unlucky, but the study's authors told LiveScience that though the data was accurate, "the paper was just a bit of fun and not to be taken seriously." Other studies have shown no correlation between things like increased accidents or injuries and Friday the 13th.

And Friday the 13th isn't a big deal in other cultures, which have their own unlucky days: Greeks and Spanish-speaking countries consider Tuesday the 13th to be the unluckiest day, while Italians steer clear of Friday the 17th. So today, try to rest a little easy—Friday the 13th may not be so unlucky after all.

Additional Source: 13: The Story of the World’s Most Popular Superstition.

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