Why Don't Spiders Get Stuck In Their Webs?

Nov 9, 2018

Why don't spiders stick to their own webs?  How do spiders walk up walls and on ceilings without falling? Why do spiders have eight legs and eight eyes? How do they make webs? And silk? What's a cobweb? How do spiders eat? And why are daddy long legs called daddy long legs when they have to have a female to produce their babies?! We're talking spiders today with arachnologist Catherine Scott.

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"How do spiders not get stuck in their own spider web?" - Nora, 5, Fort Worth, TX and Max, 7, Mesa, AZ

There are lots of different types of spider webs and not all of them are sticky. But Catherine Scott says when people think of sticky webs, they're probably picturing the orb type webs common in gardens. Those are used by spider to catch flying insects.

"The spiders that build those webs have to move around on their webs, right?" Scott says. "So while they're walking around on their webs they can just avoid the sticky lines because not every single strand of the silk in that web is sticky."

Imagine having to walk around your house being really careful about where you put your feet or you might get stuck!

But there's more to it than just being careful.

"They do have to touch the silk with their legs when they're building the web," Scott reminds us. "And they manage not to get stuck to it. So the way that they avoid getting stuck to their own silk is that their feet, which scientists call tarsi, are covered with tiny hairs that decrease the amount of surface area that makes contact with the silk. They also have a kind of non-stick chemical coating on their feet that helps the bit that does touch the silk not get stuck to it. And they're also just really careful about where they put their feet!"

"How do spiders stay on the walls and ceilings and not fall off?" - Elias, 5, Bozeman, MT and Memphis, 5, Edmonton, Alberta
Catherine Scott holding a hobo spider.
Credit Sean McCann / www.ibycter.com

Those hairy feet also play a role in how spiders walk up the walls, this time by increasing surface area that touches the wall.

Spiders that don't build webs have super hairy pads that help them stick to flat, smooth surfaces like walls.

"The spider's foot is covered with this dense tuft of hairs and each of those individual hairs is covered with hundreds of thousands more even tinier hairs. This means the spider has a huge number of tiny points of contact with the wall," Scott explains.

"The way that they stick to the wall is through something called van der Waals forces. Those forces work when two molecules are really close to each other and are attracted to each other. These are weak forces; but a lot of them together can be really strong. The attraction between millions of tiny hairs on the spider's foot and the walls allow the spider to stick. But it's also not so sticky that they can't walk around."

Here's a simpler way to think of it: "It's kind of like the spider has post-it notes on their feet that are sticky enough to stay on the wall but easy to peel off."

You can find out more about spiders on Catherine's web-site  spiderbytes.org

Click listen to hear the answers to all of your spider questions.

A tarantula shows off his hairy tarsi (feet).
Credit Sean McCann / www.ibycter.com

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