If you have a house, you probably have house spiders. They might live in your attic, basement or windowsills, or they might brazenly inhabit your houseplants. But despite their reputation as creepy interlopers, most house spiders haven't simply wandered away from home: Our houses are their natural habitats.
Some people think of spiders as insects, lumping them in with six-legged invaders like roaches or ants. But they're not insects, and they don't want to raid our cupboards. Much like their outdoor relatives that eat crop pests, house spiders just want to quietly kill the insects that do covet our food. If anything, they're on our side.
That may not help with severe cases of arachnophobia, but fear and respect aren't mutually exclusive. And the more we know about these misunderstood housemates, the less fodder we have for misguided phobias. In hopes of clearing house spiders' name, here are eight interesting facts that might persuade you to put down the shoe, pick up a magnifying glass and give peace a chance.
1. Humans and house spiders have history.
Like all modern arthropods, the spiders in your attic are descendants of 7-foot-long marine animals that lived 480 million years ago. True spiders evolved about 300 million years ago, which means they pre-date dinosaurs, not to mention humans. It may feel like they're encroaching, but they were here first.
Still, deferring to spiders on a camping trip isn't the same as sharing our homes with them. Does a spider's evolutionary seniority really give her free reign over habitats built by and for humans? Maybe not, but ousting spiders from any house is a herculean task. Not only are they stealthy and stubborn, but they've been living with us for a very long time. In fact, many house spiders are now specially adapted to indoor conditions like steady climate, sparse food and even sparser water.
"Some house spider species have been living indoors at least since the days of the Roman Empire, and are seldom to be found outside, even in their native countries," writes Rod Crawford, curator of arachnid collections at the Burke Museum of Natural History & Culture in Seattle and noted debunker of spider myths. "They usually spend their entire life cycle in, on or under their native building."
Most of the spiders you see indoors have never been outside in their lives. (Photo: Alan Levine [CC BY 2.0]/Flickr)
2. Putting a house spider outside could kill it.
Not everyone who's afraid of spiders hates them, leading many people to try non-lethal eviction. Perhaps the most common strategy involves trapping a spider in a cup and releasing it outside, where it can presumably return to its natural lifestyle. This is a noble sentiment (and often requires quick reflexes), but as Crawford explains, it may not achieve the desired result if the arachnid is a true house spider.
"You can't put something 'back' outside that was never outside in the first place," he writes. "Although some house spider species can survive outdoors, most don't do well there, and some (which are native to other climates) will perish rather quickly when removed from the protective indoor habitat. You're not doing them a favor."
In general, Crawford says, only about 5% of the spiders you see inside a building have ever set foot outdoors.
3. Not all spiders in houses are house spiders.
House spiders typically colonize new buildings via eggs sacs attached to furniture or building materials, but sometimes outdoor spiders also wander inside. Many of these are spiders that eschew webs in favor of active hunting, like wolf spiders, and may be seen scampering across floors or walls. If you release one of these outside, you might actually be doing it a favor. Just be sure to let the right one out.
Crawford notes that suspected "wolf spiders" are often just male European house spiders, which tend to roam around more than females do. And although many house spiders weave webs, a few mix things up by actively hunting prey. It's not always easy to tell indoor and outdoor spiders apart, but it might help to study the eyes more than markings or other features. For reference, compare this photo of a common house spider with this photo of an American wolf spider.
And for more help, check out MNN's guide to different types of spiders.
Tegenaria domestica is known as the domestic house spider or barn funnel weaver. (Photo: Nikk [CC BY 2.0]/Flickr)
4. Not all house spiders look alike.
To complicate matters further, house spiders come in lots of shapes and sizes. The types in your house depend largely on where you live, although humans have helped many species spread around the planet, especially those from Europe.
One of the most abundant house spiders is Parasteatoda tepidariorum, aka American house spider, which is native to North America but now found around the world. Measuring 4 to 8 millimeters long, these yellowish-brown spiders have a tall, round abdomen and two rows of four eyes. They build tangled webs, often both outside and inside a building, so evicting them may be harmless — and futile. On the bright side, they have relatively mild venom and bite humans only in self-defense.
Another widespread species is Tegenaria domestica, aka domestic house spider, which is native to Europe but has also become cosmopolitan with human help. It first appeared at U.S. shipping ports in the 1600s, and is now found across most of North America as well as Europe and western Asia. It ranges from 6 to 12 mm in length, with a reddish-brown "head" (the cephalothorax) and a pale, speckled abdomen. It builds funnel-shaped webs, and is known to prey on pest insects inside homes.
Steatoda grossa, aka cupboard spider, has similarly expanded far beyond its native Europe, including North America and Australasia. Varying in length from 4 to 11 mm, this spider is known for messy webs that contribute to indoor cobweb buildup. It's also one of several Steatoda species known as a "false black widow" because people commonly confuse it with that highly venomous spider. Not only does it lack the black widow's red hourglass, however, but its bite is more like a bee sting.
Other common house spiders include Badumna insignis (black house spider, native to Australia and New Zealand), Pholcus phalangioides (cellar spider, cosmopolitan), Cheiracanthium mildei (yellow sac spider, cosmopolitan), Eratigena atrica (giant house spider, Europe and North America), Eratigena agrestis (Hobo spider, Europe and North America) and Kukulcania hibernalis (Southern house spider, Americas).
5. Spiders don't use plumbing to sneak inside.
Since spiders are often found trapped in sinks or tubs, many people assume that's how they got inside. But modern drains feature sediment traps that would prevent spiders from passing, Crawford points out. "I don't know of even one case where a spider was actually shown to migrate into a house through plumbing."
Instead, he adds, they probably just got stuck while looking for water. "House spiders are thirsty creatures living in a very water-poor environment, and any that venture near a sink or tub with drops of water in it will try to reach the water, often by climbing down a wall. Once in the slick-sided porcelain basin, they are unable to climb back out unless a helpful human 'lends them a hand.'"
There's no need to fear false black widows, but also no need to pick them up. (Photo: promiseminime [CC BY 2.0]/Flickr)
6. House spiders pose very little danger.
Spiders in general don't deserve their scary reputation. They rarely bite people, and even when they do, most species' venom causes only moderate and short-lived effects. That's true for the vast majority of house spiders, which have no incentive to bite anything they can't eat unless they think it's a matter of life or death.
"House spiders prey on insects and other small creatures," Crawford writes. "They are not bloodsuckers, and have no reason to bite a human or any other animal too large for them to eat. In any interaction between spiders and larger creatures like humans, the spiders are almost always the ones to suffer."
7. In fact, house spiders can be helpful.
As mentioned earlier, spiders are a potent defense against agricultural pests like aphids, moths and beetles. House spiders offer similar benefits indoors, helping suppress a wide variety of insects without the need for synthetic insecticides.
"Spiders feed on common indoor pests, such as roaches, earwigs, mosquitoes, flies and clothes moths," explains a fact sheet by Bayer CropScience. "If left alone, spiders will consume most of the insects in your home, providing effective home pest control." And by keeping these populations in check, spiders can even help limit the spread of disease carried by insects like fleas, mosquitoes and cockroaches.
If you want to make sure your house spiders are pulling their weight, check in and under their webs to see what they've been eating. Many web-dwelling house spiders simply drop the remnants of their prey to the floor after eating, which can make an annoying mess but also provide evidence of their contribution to the household.
8. There are humane ways to manage house spiders.
If you still can't stand house spiders, it is possible to keep them in check without losing your cool. Instead of resorting to pesticides, smashing or other potentially lethal methods (like a vacuum cleaner), try to stay ahead of population booms by limiting suitable habitats. Check windows, eaves and other popular spider hangouts, and remove any cobwebs you find. This probably won't eliminate your house spiders, but it might drive them to lower-profile haunts like a shed, garage or crawlspace.
Sealing potential entry points may not affect house spiders, since they don't sneak in from outside, but it could limit incursions by other spiders. And if it also prevents insects from getting inside, it may indirectly reduce your house spiders by limiting their food supply. Various myths suggest house spiders are repelled by osage orange, horse chestnuts or even copper pennies, but Crawford is doubtful.
In many cases, house spiders are like Michael Jordan: You can't stop them; you can only hope to contain them. So rather than trying to play defense against such a resilient force of nature, why not just sit back and marvel at them? It'll make life easier for everyone — except for that fruit fly buzzing around the kitchen.