Why do spiders like high-rise buildings and how do they get up there?

Tall buildings are better habitats for spiders than you might think. Here’s why these arachnids make these buildings home.

Curious City spiders thumb
A spider climbs the Willis Tower in downtown Chicago in 2008.
Curious City spiders thumb
A spider climbs the Willis Tower in downtown Chicago in 2008.

Why do spiders like high-rise buildings and how do they get up there?

Tall buildings are better habitats for spiders than you might think. Here’s why these arachnids make these buildings home.

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While Nina Fletcher was working as an accountant on the 32nd floor of 875 N. Michigan Ave. (formerly the John Hancock Center), she made an unlikely friend. She would watch a spider outside of her window and while her co-workers found the spider a little gross and creepy, she did not. She even gave him a name: Henry.

“We are on the same team. They (spiders) get rid of bugs and I hate bugs,” she said.

Nina would observe how “Henry” the spider built his web and ate bugs and began to wonder what his life was like.

“I really wanted to know if my 32nd floor spider had spent his entire life as a 32nd floor high-rise spider.”

And Nina is not the only one that’s asked Curious City about these arachnids (by the way, spiders are not insects). Other listeners have asked: Are these spiders poisonous? Why do they seem to like Chicago’s tall buildings? And how do they even make their way up these super tall structures?

So to get some answers we turned to Petra Sierwald, an associate curator in the division of insects at the Field Museum.

Here’s what we learned:

Bridge spider back
A bridge spider that was collected from a Field Museum window in 1953. Stephanie Ware / The Field Museum of Natural History

What kind of spider lives on high-rise buildings in Chicago?

If you see a spider up on a high-rise building in Chicago, it’s likely what’s commonly known as a bridge spider, Sierwald said. The bridge spider is a type of orb weaver spider. It makes orb-shaped webs, those round, spiral patterned webs you likely picture when you think of spider webs.

Sierwald said you can find these types of spiders all over the Northern Hemisphere. Their natural habitat is rocks overhanging water, but they’re also often found in manmade environments, in urban areas like Chicago. Though these spiders aren’t particularly eye-catching to the naked eye, they get more interesting when you can zoom up close.

“They’re actually kind of dull looking [but] under the microscope, you will see quite intricate patterns of black and gray and brown on the abdomen,” Sierwald said.

Their population is particularly large in Chicago because they don’t have a lot of predators here, like birds, lizards or snakes.

Spiders are generally known for their strong silk, and for the bridge spider, it’s no different. Their webs aren’t water soluble, so it’s tough to get rid of them. Instead of spraying them, you would need to scrape their web off a window, but there’s no need since they’re totally harmless.

Although these spiders usually stay outdoors, Sierwald says that if you find one inside your home, it’s probably a male spider who got lost looking for a mate.

Chicago skyline
The Chicago skyline as seen from a helicopter in 2012. Carolyn Kaster / Associated Press

What makes tall buildings a good home for bridge spiders?

High-rise buildings, like houses and other buildings around Chicago, offer spiders a few things they’re looking for.

First, tall buildings are heated, which means the spiders are able to survive on them even during the winter.

“What is beneficial to them being on these buildings is that they can be active much longer during the year,” Sierwald said. “There’s always a little warm crack or nook somewhere next to the window sill where they can basically get protection from the wind during the night.”

Besides the heat, skyscrapers and apartment buildings also provide light, which lures in bugs. This means that spiders have a plentiful food source if they set up their webs in front of a window. In Chicago, there are many skyscrapers and tall buildings, so there are a lot of places for spiders to set up their webs.

“This is the ideal place for them. It must be paradise compared to their natural situation because we provide the light that attracts insects, so they just eat very well,” Sierwald said.

How do bridge spiders get up skyscrapers?

It’s a long climb from the ground to the 32nd floor of the John Hancock Center, but spiders can move around with the help of the wind. The process is called ballooning, and it’s a bit like hang gliding with the help of a strand of silk.

“They fly like a kite,” Sierwald explained. “They have the spinnerets at the end of the body. They let out silk, and depending on the length of the thread, and how they spread their eight legs, they can catch an uplifting current and basically fly.”

Spiders can also move down a skyscraper by decreasing the length of the silk thread, then floating down. Ballooning isn’t unique to bridge spiders. According to Sierwald, most spiders can begin ballooning when they’re young.

Ballooning can take a spider long distances. Sierwald says that spiders have even been found on ships crossing the Atlantic Ocean, typically around the mast of the ship.

Do bridge spiders spend their whole lives up on the windows of high-rises?

Once a bridge spider makes its way up a high-rise building, it could very well stay up there for the rest of its life — especially if they’ve found a good site to build their web where there are lots of insects.

John Hancock spider
A spider as seen at 875 North Michigan Avenue, also known as the John Hancock building. Uploaded by user Wallyp1982 / Reddit
Sierwald said that in Chicago’s climate, spiders can live around two years, so your window spider could become a regular companion.

Spiderlings are even born on top of high-rise buildings, and the little nooks and crannies spiders hide in also make great places to keep their egg sacs. Spiderlings also balloon, and may leave to find another place to set up their web so they have a good supply of food instead of competing with other spiders for every meal.

Although spiders might move locations if they’re chased away by a larger spider, once they’re up there, they won’t typically go to ground-level. If a spider comes down, it would be by accident, and Sierwald said that there’s no fear of falling off the building entirely.

“Spiders never do anything without a seatbelt. They’re always attached with a dragline strand to the building,” she explained.

This also applies to when spiders are fighting for the best spot to make their web. If one spider gets knocked down, it’ll still be hanging on the side of the structure with a thread of silk, but it’ll have to find another place to spin a web and catch dinner.

Spider in Japan
Aaron Favila / Associated Press

What’s the best way to spot these spiders hanging out on highrises?

Now that it’s getting cold, these spiders will likely be a little harder to see since they’ll be burrowed away in cracks and crevices. But even in the winter, you’ll see hints that bridge spiders are outside your window.

“I may look for silk, and I may find a silk thread,” Sierwald said. “Watch what they are doing, and watch for where the males come in knocking.” If you see one spider tugging at a strand of silk, it could be a male trying to get the attention of a female spider.

Aside from high-rises, another good place to look for bridge spiders are ballparks like Soldier Field. Ballparks also have plenty of places for spiders to tuck themselves away during the winter, and Sierwald said the lights give them another ideal location to set up a web.

But when it comes to the peak time to do some spider watching, late summer is your best bet. With more insects flying around, this is the time of year when bridge spiders are the biggest.

“Always leave the light on in the summer at night. At least for one or two hours to provide good food for them,” Sierwald said.

Nina curious city spiders
Photo courtesy Nina Fletcher

More about our question-asker

Nina Fletcher has lived in the Chicago area for 14 years, and currently lives in Bedford Park. Her interest in spiders first started when she spotted a large group of them while having dinner in Lincoln Park with a friend.

“We were sitting next to a big picturesque window at sunset, and the lights went on, and as we sat there, we watched about two dozen spiders make their webs. They biggest one at the top while the other ones would compete all the way down,” Fletcher said.

Spiders included, she’s a big animal lover. Fletcher lives with her husband, Brad, and their cat, Nucky. She also said, “the love of my life was a gecko named Stevie Wonder.”

Thanks also to Will Ramsey, Jane Harding, Irene Stemler, Mike DiGirolamo, Kate Watson Moss and Meredith Young for their questions about spiders in Chicago.

Sophia Lo is Curious City’s multimedia intern. Follow her @sophiamaylo.