Giant House Spider — The Big One in the Bathtub

What you are looking at

If you are reading this in September with a large brown spider in front of you, this is almost certainly the animal. The giant house spider produces more alarmed phone calls in Whatcom County than every other arthropod combined, and it is harmless.

WSU Extension’s fact sheet gives it a body length of half an inch to an inch, with the largest adults reaching around four inches across the legs. Brown to grayish-brown, with legs that are somewhat hairy rather than glossy. Coloring and markings vary a great deal between individuals, which WSU flags as the reason appearance alone is a poor way to identify it.

Four inches of leg span moving quickly across a floor at eleven at night is a genuinely startling thing, and we are not going to pretend otherwise. The reaction is proportionate to the impression. It is not proportionate to the animal.

WSU’s own summary of the risk: the bite of the giant house spider is not known to pose any threat to humans.

Where it lives

This is a western Washington animal specifically. WSU records it as common in Washington homes west of the Cascade Mountains, along with the coastal regions of Oregon and British Columbia, and absent from the country east of the Cascades. Whatcom County sits comfortably inside that range.

It arrived from Europe, like most of the spiders that do well inside northern European-style housing, and it has done extremely well here — wet, mild, never hard-frozen, and full of buildings with crawlspaces.

The naming is unsettled, which is worth a sentence because you will see three different names for the same spider. WSU’s fact sheet lists it as Eratigena atrica, noting the earlier names Tegenaria gigantea and T. duellica. The Eratigena atrica grouping has since been split again by some arachnologists, with E. duellica treated as the western species. We list it here as Eratigena duellica, and either name refers to the animal in your bathtub. Nothing about the identification, the risk or the treatment turns on which label wins.

The web, and where the spider sits

It is a funnel weaver. The web is a flat, roughly horizontal sheet of silk with a tube at one edge where the spider waits. Insects that blunder onto the sheet get tangled long enough for a very fast animal to cross it and take them.

That web is not sticky. It works on entanglement and speed, which is why the spider is built the way it is. The sheets accumulate dust and become obvious in corners of garages, sheds, crawlspaces and unfinished basements, behind stored material, in window wells, under decks and along the outside of foundations.

WSU notes something that surprises people: this species is not an especially active climber. Indoors, they mostly stay at floor level, which is why they end up stuck in sinks and bathtubs. It also explains why so much of what reduces them is work done at ground height — thresholds, sweeps, foundation gaps — rather than anything happening at eaves level.

They run away very quickly when startled, and they are not aggressive. The speed is the whole defense.

Why they appear all at once in autumn

WSU puts peak visibility at mid-summer to early fall, when males in particular are highly mobile. That is the mechanism, and it is worth stating precisely because nearly every intuitive explanation for it is wrong.

Mature males stop maintaining a web and start walking, looking for females. They move at night, they cover ground, and they go through buildings rather than around them. The Burke Museum, whose arachnology collection is the regional reference, records that the seasonal rise in sightings coincides with the mating season of the species involved and that what people are seeing is sexually mature males wandering in search of mates.

Not spiders escaping the cold. Not spiders sensing winter. Not an infestation establishing itself. A mating season, which starts on a schedule and finishes on one.

The Burke makes a second point that runs against almost everything published on this subject, including some of what the rest of this industry says, and it is worth passing on honestly: house spiders are largely a distinct indoor population rather than garden spiders that came in. Generally fewer than five percent of the spiders seen indoors have ever been outdoors at all. Under that reading the male crossing your kitchen did not enter the house in September — he has lived in it, or in the crawlspace under it, since he hatched, and he only became visible when he started moving.

Both readings point the same direction for a household. The spiders were already there, the season is short, and it ends whether or not anything is sprayed.

More spiders means more insects

The single most useful thing to understand about spider numbers is that they are downstream of something else.

Spiders are predators with no ability to increase their own food supply. A building supporting a lot of them is a building supporting a lot of insects, and the insect population is set by things that have nothing to do with spiders: exterior lighting drawing flying insects to the walls all night, damp ground and poor drainage producing them in volume, vegetation touching the siding, a wet crawlspace acting as a nursery, standing water, and open routes that let all of it inside.

So when the count goes up year over year, the useful question is what changed about the food, not whether something is breeding in the walls. It very rarely is. Egg sacs do get laid indoors in undisturbed spaces, and clearing them is worth doing — but a house does not fill with spiders because spiders had offspring in it. It fills with spiders because it is a good place to hunt.

That is also why the interventions that work are unglamorous. Change the porch light to a warm color temperature or put it on a motion sensor and you take a share of the food away. Dry the crawlspace and you take away more. Pull the shrubs back off the wall and you remove both a habitat and a bridge. None of that is a treatment, and all of it outperforms one over a season.

What it eats, and what it competes with

A giant house spider takes whatever walks or blunders onto its sheet: flies, small moths, beetles, ants, sowbugs, silverfish, midges, and a steady stream of the smaller arthropods that live in a damp Pacific Northwest crawlspace. In a garage or an outbuilding this adds up to real volume across a season.

It also eats other spiders, including smaller funnel weavers, and this is where the animal earns its place in an honest account. UC IPM records that in parts of the Pacific Northwest this species has competitively displaced the hobo spider — the one with the reputation. Whatever the mechanism, the practical shape of it is that where giant house spiders are well established, the spider people are actually afraid of is scarcer.

Which produces an awkward result for the standard version of this page. Killing every large brown spider in a basement removes the animal that was suppressing the population you were worried about in the first place. We are not going to tell anybody they have to live with a four-inch spider on principle. But the trade-off is real and it is worth knowing before the decision is made with a shoe.

Egg sacs and the quiet part of the year

The season most people never think about is the one where intervention is cheapest.

Females that mated during the autumn walk produce egg sacs — pale, papery, and tucked into exactly the undisturbed places the webs are: the back corners of a crawlspace, behind stored boxes in a garage, under the lip of a shelf, inside a window well, in the folds of a tarp that has not moved since spring. They sit through the winter, and the spiderlings emerge as the year warms.

Removing an egg sac in February prevents a population rather than reducing one, and it takes a vacuum and ten minutes. Removing the spiders it produced in September takes considerably longer and works considerably less well. The whole economics of spider work in this county sits in that comparison, and almost nobody acts on it, because in February nothing is visibly wrong.

November through March is also when the physical work is easiest to schedule and when clearing webbing from a crawlspace or a shed actually holds, because nothing is rebuilding it immediately. If a household has decided to do something about spiders, that is the half of the year to do it in.

Telling it from a hobo spider

This is the question the page exists to answer honestly, and the honest answer is uncomfortable.

You cannot do it by eye, and neither can we. WSU Extension’s identification factsheet states that unless you are able to examine the very small reproductive structures, you cannot definitively identify a hobo spider. UC IPM says the same: separating funnel-weaving species means examining the genitalia, and that is work for a qualified arachnologist with a microscope and a mature specimen. Juveniles cannot be determined at all, because the structures have not developed.

What is being examined is the retrolateral tibial apophysis on a male’s pedipalp — a projection that in the hobo spider is single and squared off on top — or the epigynum on a female, where the hobo carries a large central cavity ringed with thick raised ridges. These are millimeter-scale features. They are not visible in a photograph of a spider on a wall, and they are not visible to an experienced technician standing in a basement.

The one field character that does something. Turn the spider over, through the base of a clear container, and look at the sternum — the plate on the underside between the leg bases. WSU’s key: the giant house spider and the barn funnel weaver carry three or four pairs of light spots along the sides of it, while the hobo spider carries a light stripe running down the middle. WSU’s own shorthand is that if you see spots, then a hobo it is not. That is a genuine character and it can be seen without magnification on a good specimen. It is still not easy on a live animal that moves the way this one moves.

The tests that do not work, and it is worth being blunt about each:

Chevrons on the abdomen. The most widely circulated hobo spider test online. Neither WSU’s identification key nor UC IPM’s pest note on the species uses it at all — both skip straight to the reproductive structures. A character the two authorities on identifying this spider do not bother to mention is not one you can use at a kitchen table.

Leg banding, run forwards. WSU does use banding, but only in one direction: visible dark rings around the legs mean it is not a hobo spider, because hobo legs are uniformly colored. That is a valid way to rule the species out. Running it the other way — plain legs, therefore a hobo — is not valid, because a great many harmless funnel weavers also have plain legs. An exclusion test used as an inclusion test produces confident wrong answers, which is exactly what has happened across this region for thirty years.

Size. Overlapping, but the odds point away from alarm. The giant house spider is the larger animal, so a really big frightening spider is an argument against the hobo rather than for it.

Color. WSU calls coloration one of the least reliable characteristics available, and specifically warns that non-specialists reach for it first.

Why the identification matters less than it feels like it should

Here is the part that a fear-based version of this page could never print.

Work through it. The two species are difficult to separate without a microscope. The giant house spider is harmless. And the hobo spider — the one you cannot rule out — is not dangerous either: the Washington State Department of Health states that hobo spiders are not considered dangerous as previously thought, and that there is no evidence their venom has caused skin necrosis in people.

If both branches of the uncertainty end in a harmless animal, then the uncertainty does not carry any risk. You can fail to identify the spider in your bathtub and lose nothing by it.

The identification still matters, for two reasons that have nothing to do with danger. It matters for honesty, because a company that names a species confidently from across a room is telling you something it does not know. And it matters because getting it right is how the correction eventually reaches everyone — every time somebody learns that the big one is the harmless common one, the story about the rare frightening one loses a little ground.

WSU’s abundance figure is the useful number to carry away: west of the Cascades, giant house spiders outnumber hobo spiders roughly three to one. UC IPM records that the larger species has competitively displaced hobo spiders in parts of the Pacific Northwest. The base rate is already on the reassuring side before anybody picks up a hand lens.

What a bite would be, if you managed one

People ask, so: WSU’s position is that the bite of this species is not known to pose any threat to humans, and that is the whole medical entry.

Getting bitten by one is also harder than it sounds. The Burke Museum’s arachnologist, having handled tens of thousands of spiders across a forty-four-year career, records three bites in that time, all with trivial consequences, and observes that these animals are not bloodsuckers and have no reason to bite something far too large to eat. A spider that hunts by ambushing insects from a funnel gains nothing from confronting a person.

The bites that do happen are compression events — an animal pressed against skin inside a sleeve, a glove, a boot or a bed, responding the way anything responds to being crushed. Shaking out gloves and footwear left in a garage or shed is sensible for that reason and for several others.

The more useful warning runs the other way. The Washington State Department of Health notes that unexplained wounds are often misdiagnosed as spider bites, and that the misdiagnosis can leave the real condition untreated. If a wound worries you, the answer is a physician promptly, whatever you think put it there. We identify spiders; we are not qualified to read skin, and a pest control company that offers to is overreaching.

The one in the bathtub

Practically, the most common encounter in this county.

It walked in across the floor at night, went over the rim, and found the glazed surface too slick to climb back out. Poor climbers, remember. The tub is a pitfall trap it stumbled into, not a route it used, and it has probably been circling in there for hours by the time anybody finds it.

Drape a hand towel over the edge and it will let itself out. If you want it gone now, a glass over the top and a piece of stiff card slid underneath moves it without anybody touching anything.

The Burke Museum raises a point worth knowing before the automatic decision to put it outside: true house spiders are an indoor lineage, and you cannot put something back outside that was never outside to begin with. Some will not do well out there. That is not an argument for keeping it in the bathroom — it is just a reason not to feel that the outdoors is where it belongs.

Killing it is a choice rather than a necessity, and the trade-off is real. These are territorial predators that eat other spiders and a steady volume of insects. The one you remove was suppressing something.

What it means for a Whatcom County house

Local housing stock shapes this more than the species does.

Older Bellingham — the Lettered Streets, Sunnyland, Columbia, York, South Hill. Unsealed crawlspaces, original window frames that have moved, and a hundred years of settlement gaps. The crawlspace under this housing is effectively a reservoir, and the living space above it gets restocked from below all year.

Anywhere against a treeline — Sudden Valley, Silver Beach, the Whatcom Falls corridor, Edgemoor. Heavy insect pressure off the woods and the water means high spider numbers no matter what anybody does to the siding. Lighting choices carry more weight here than anywhere else in the county.

Newer construction on the hill neighborhoods is better sealed, and the complaint shifts from spiders indoors to webbing accumulating on eaves, entries and outdoor living space.

Rural properties around Lynden, Everson, Nooksack and Sumas carry large populations in barns, equipment sheds and outbuildings, where the spiders are doing genuinely useful work on flies and nobody benefits from removing them.

Seasonal homes around Blaine, Birch Bay and Semiahmoo stand empty through the wet months and accumulate undisturbed webbing, so the spring visit is usually a cleaning job rather than a treatment.

Across all of them, the specific openings that matter are at floor level: the sweep under a garage side door, sliding door tracks that are never sealed at the ends, window wells that have filled with leaf litter, the crawlspace access hatch, and the holes cut for water, gas, power, cable and heat pump lines.

Every one of those is also a rodent route and a heat-loss path, which is why we would rather sell somebody an afternoon of sealing than a subscription.

Our verdict, and why

Rarely worth treating. Not never — but rarely, and not because of the species.

There are three situations where we think the work is genuinely worth doing. A garage, shed or crawlspace with a heavy standing population in a space the household actually uses, where clearing webbing and egg sacs and dealing with the conditions makes a visible difference. A commercial site with a legitimate appearance standard. And a household where somebody’s fear of spiders is affecting how they live — which is a real quality-of-life problem, not a silly one, and it changes the priorities without changing the methods.

Outside those, the honest recommendation is a door sweep, a warmer porch light, a drier crawlspace and a broom.

That is very close to what the regional extension services recommend when they have nothing to sell. Oregon State’s guidance is to remove spiders found indoors and leave the ones outdoors alone, with catch-and-release as the primary answer and chemical control unnecessary. WSU’s management notes for house-dwelling funnel weavers lead with mechanical removal, sealing openings and checking firewood, and observe that chemical control is temporary without prevention.

We can spray a foundation line. It is simply not the thing that decides how many of these you meet next September.

Send us a photograph

Identification costs nothing and it settles most questions in a couple of minutes. Photograph from directly above with a coin or a fingertip in the frame, and from the side if you can. If the spider is in a clear container, a shot through the base showing the underside is the single most useful angle, because that is where the sternum is.

We will tell you what we think it is. Where the answer is that it cannot be resolved without magnification, you will hear that instead of a guess, and you will hear at the same time that it does not change anything about how safe you are.