A house spider at rest indoors. Photo: Matt Bertone, CC BY-ND
The short answer: almost every spider you find indoors is harmless to people, quietly eats insects you would rather not have around, and would much prefer to avoid you. Squashing it removes a free pest controller and does nothing to stop others living out of sight. If you truly cannot share a room with one, catching it and putting it outside works better for everyone.
That is the case made by Matt Bertone, an entomologist at North Carolina State University who has studied the creatures living inside ordinary homes. Below is what the research shows about house spiders, why they are useful, which few deserve caution, and how to live with them calmly.
Spiders already live in every home
Most of us picture our houses as sealed off from nature. Biologists see them differently: a house is a habitat, with its own food chain. In a 2016 study published in the journal PeerJ, Bertone and colleagues searched room by room through 50 houses around Raleigh, North Carolina, and catalogued the arthropods they found. Spiders turned up in every single house.
Two groups were especially common. Cobweb spiders (family Theridiidae) build untidy, tangled webs in corners, behind furniture and under shelves. Cellar spiders (family Pholcidae), the long-legged, fragile-looking ones often called daddy long-legs, hang in loose webs in basements, bathrooms and ceilings.
Some indoor spiders wander in by accident and do not last long. Others are short-term visitors. A good number, though, spend their whole lives indoors, feeding, mating and laying eggs without ever going outside. If you have not seen one this week, that does not mean your home is spider-free; it simply means they are doing what they do best, staying out of sight.
Free pest control
Spiders are generalist predators: they will eat almost anything they can overpower. Indoors, that usually means flies, moths, small beetles, silverfish and mosquitoes, plus other spiders. A healthy population of house spiders is one of the reasons you are not overrun by small insects.
Some species are remarkably specialised. Around Lake Victoria in Kenya and Uganda, a small jumping spider called Evarcha culicivora lives in and around houses and actively selects female Anopheles mosquitoes that have just fed on blood, the very insects that spread malaria. Research led by Robert Jackson and Ximena Nelson, published in 2005, showed it picks these mosquitoes over other prey.
Cellar spiders have their own tricks. Although they mostly eat insects, they will also invade the webs of other spiders and pluck the silk to imitate a struggling insect, luring the owner close enough to be caught. They are known to prey on spiders that people find far more worrying, including redbacks and house spiders larger than themselves.
A cobweb spider wraps an insect that blundered into its web. Photo: Matt Bertone, CC BY-ND
Are house spiders dangerous?
Nearly all spiders use venom to subdue prey, but for the vast majority of species it is far too weak to harm a person, and many have fangs too small or weak to pierce human skin. Spiders do not hunt people. When disturbed, their instinct is to run, freeze or drop away on a thread. Bites mostly happen when a spider is pressed against skin, for example when it is trapped inside clothing or a shoe.
Arachnologists also point out that many skin wounds blamed on "spider bites" turn out, on examination, to have other causes such as bacterial infections. Unless someone actually saw the spider bite, the label is often a guess.
There are exceptions worth knowing about. Widow spiders and recluse spiders are considered medically important in parts of the world. According to the US CDC, widow bites can cause spreading pain and muscle cramps, while recluse venom can damage skin tissue around the bite. Even so, bites from these species are uncommon and rarely serious. If you think one of them has bitten you, or a bite becomes painful, spreads or causes trouble breathing, see a doctor.
Common spider myths
"Daddy long-legs are the most venomous spiders, but their fangs are too short"
This old story is false on both counts. Cellar spiders can bite, but their venom is not medically significant to humans. In a 2004 test on the television show MythBusters, a bite produced only a mild, brief burning feeling.
"Spiders come indoors when it gets cold"
Most spiders you meet indoors already live there. The sudden autumn sightings are often adult males roaming in search of mates, which makes them more visible, not more numerous. Putting an outdoor garden spider into a warm house is usually not doing it a favour.
"We swallow spiders in our sleep"
This claim is widely repeated online, but there is no evidence for it. A sleeping person breathes, moves and vibrates in ways that a spider reads as danger, and it has every reason to keep away.
If spiders frighten you, you are not alone
Fear of spiders is one of the most common phobias. Many legs, sudden movement and the chance of a bite add up to something our brains are quick to flag as a threat. Even professional insect scientists are not immune: an article in the journal American Entomologist described entomologists who are themselves arachnophobic.
The good news is that fear can ease with knowledge and gentle exposure. The first video below follows an arachnologist who grew up terrified of spiders and ended up devoting a career to them. The second is a short nature clip shared by Brut Nature.
An arachnologist on growing up afraid of spiders and coming to find them fascinating.
How to live with house spiders
- Leave them be where you can. A spider in a high corner, behind a bookshelf or in the basement is doing pest control at no cost.
- Use the cup-and-card method. Place a glass or jar over the spider, slide a stiff card underneath, and release it outdoors, ideally near shrubs or a wall.
- Cut down on what attracts them. Spiders follow prey. Fewer flies and moths, through screens and less outdoor lighting near doors, means fewer spiders.
- Vacuum webs, not whole populations. If cobwebs bother you, remove them in living spaces and let spiders stay in less-used areas.
- Skip the bug bombs. Broad insecticide sprays expose your household to chemicals and tend to kill the useful predators along with the pests.
- Take care in storage areas. Where widow or recluse spiders occur, wear gloves when handling boxes, firewood or clothes left in garages and sheds.
Spiders have shared human homes for as long as there have been homes. Seen for what they are, small and shy predators working the night shift, they are much easier to live with. Next time one crosses the floor, it is worth considering letting it go on its way.
Sources: Research and photos by Matt Bertone, North Carolina State University; Bertone et al., PeerJ (2016); Jackson, Nelson and Sune, PNAS (2005); Wikipedia: Evarcha culicivora; Wikipedia: Pholcidae; CDC NIOSH: Venomous spiders.
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