Simone M. Scully

If someone tells you to picture a spider in your mind, chances are you’ll think of a big hairy one, like a tarantula, crawling across a desert rock.

But the truth is, spiders don’t just live in warm places. Worldwide, there are over 40,000 species of spiders and they’re found living on every continent of the world except Antarctica. That means that they can survive in a wide range of climates.

“Spiders are some of the northernmost animals and are a dominant predator even in the high Arctic,” explains Michael Draney, a biology professor at the University of Wisconsin-Green Bay who has been studying spiders since 1987.

In fact, he says, there’s even a type of spider that lives on Mount Everest: “It’s a jumping spider called Euophrys omnisuperstes,” he explains. “The species names means something like ‘above it all.’ It has been found on the slopes of Everest at 6,700 meters [or 22,000 feet above sea level], making it the highest permanent land animal out there. It seems to feed mainly on insects that get blown up there and deposited on the snow or rock surface.”

But even closer to home, spiders can survive some pretty cold, snowy winters — and, unlike their Mount Everest cousin, some are able to survive changing seasons, from warm summers to frigid winters.

So how do they do it? How do they manage to keep themselves from freezing or running out of food when the air grows older, the days get shorter, and the snow starts to fall?

Well, to start with, they’re poikilothermic, or “cold-blooded.”

This means that unlike humans and mammals that need layers of insulation to tolerate a dip in temperatures, spiders don’t really get uncomfortable when they’re cold. They don’t shiver and they’re not attracted to warmth because “their body temperatures can vary with the environment without doing them any harm,” explains Rod Crawford, spider expert and arachnid curator at the Burke Museum in Washington.

This means that, when it gets cold, they become less active and eventually, they become “dormant” or enter what scientists call “diapause” — a hibernation-like state. It also means that they need less prey in the winter.

“Their metabolism is related to temperature,” explains Draney, “so they need less food at cold temps, plus their digestion is slower.”

Of course, being cold-blooded doesn’t mean spiders are completely immune to freezing — after all, they can’t survive in Antarctica.

That’s why spiders that are native to colder climates have a special adaptation: they can prep their blood to keep it from freezing.

When the temperatures start to drop, their bodies start producing glycol and protein compounds in their hemolymph (the spider’s version of blood) that act like antifreeze. As these compounds build up in their cells, they lower the freezing point of those cells, allowing them to survive at much lower temperatures before freezing.

“[A spider] has to get at least to five degrees celsius — below freezing — to freeze,” explains Crawford, and sometimes, it’s even lower.

For example, according to Susan Anthony, a PhD student at the University of Western Ontario who is studying the effect of low temperatures on spiders, there is one type of house spider, called Parasteatoda tepidariorum, that enters diapause in the winter as a juvenile and, thanks to the compounds in its blood, can survive in temperatures as low as minus 20 degrees Celsius — or minus 4 degrees Fahrenheit — without freezing.

Still, as anyone who’s wandered around in the snow can probably tell you, it’s not like spiders just hang out in the open when temperatures dip below freezing.

“Spiders don’t really migrate,” explains Draney. So instead, “all northern spiders must overwinter” — i.e. find a safe place to hide until spring comes.

How they overwinter, though, depends on the species. Just like not all species of spiders build webs to catch prey (some, like wolf or jumping spiders, actually hunt their prey, while others, such as trap door spiders, lie in wait), not all spiders spend the winter months in the same way.

Some seek winter shelter in places where temperatures remain a little warmer than outdoors, such as in leaf litter, rock piles, building cracks and under loose bark. To help block cold wind, some will even build themselves a little pod with their silk, enclosing themselves until it is warm enough to come out and hunt.

“Few overwinter as eggs,” says Draney, “[because] it seems that eggs are prone to freezing.” However, spiders that reproduce in the fall, like most orbweavers, have spiderlings that develop and hang out in their egg sacs until spring when it’s time to emerge.

Others reproduce in the spring, such as sheet web spiders or Linyphiids, and have spiderlings that feed and grow all summer long, explains Draney. This means that come winter, they have to overwinter as adults — and a good place to do this is actually underneath the snow in what scientists call the subnivean zone, a cave-like space that opens up between the soil and the snow surface because of geothermal heat. The temperatures never drop much below freezing here. In fact, some species that live under the snow even mate and reproduce during this time — which is advantageous for them because there are fewer predators around.

The good news is that most outdoor spiders do not try to come into your home for the winter. This is one of the biggest myths about spiders. According to Crawford, only about 5 percent of spiders that you see indoors have ever been outdoors. Because they’re cold blooded, they aren’t drawn to your warm home — and in fact, if they end up inside your house for some reason, they’ll likely die, or at least, fail to reproduce. So if you see a spider in your home in the winter, it’s probably always been there; you just didn’t see it before.

This does mean that if you see a spider inside your house and want to be kind, it’s best not to put it outside — at least not when it’s cold out. “Some non-native spider species, [such as cellar spiders], only live in our houses and can’t survive the winter outside,” explains Draney. “Other species — like the northern house spider Steatoda borealis — can survive indoors or out, but if you throw one outside in the winter, it is doomed because it is not acclimated with antifreeze.”

“Spiders need to be gradually exposed to nonlethal cold temperatures over several days to build up cold tolerance,” he continues, “so unfortunately the spiders don’t give homeowners a lot of options. Either tolerate them or kill them.”

If the idea of leaving them inside leaves you a little squeamish, though, just remember: most of them are harmless and prefer eating bugs to bothering you. Plus, says Crawford, they help keep your house clear of other insects, such as cockroaches, clothes moths, stored-food pests, booklice, fleas, bed bugs, and carpet beetles.

If that’s not enough to convince you, consider this: for every spider you see, there is probably a hundred others you don’t. “Stepping on one individual changes nothing — except your karma,” says Crawford. “Just wave as they go by.”