Tegenaria domestica, the common house spider that is believed to have been carried across the Atlantic by European settlers, spends its entire life indoors, and the individual crouched behind a bookshelf in Ohio tonight is descended from a lineage that has likely not touched wild soil in many thousands of years. These animals are obligate synanthropes, meaning they need human structures the way a barnacle needs a rock. Put one out in the garden as an act of mercy and it will almost certainly be dead within days.
The intuition that spiders drift in from outside on the first cold night of autumn is one of the most durable folk beliefs in domestic natural history. It is also mostly wrong. The spider under the radiator was born under the radiator. So was its mother.
The spider that forgot the outdoors
Tegenaria domestica belongs to a small club of arthropods that have gone through what biologists call domiciliation, the slow evolutionary drift toward living inside human buildings. The pharaoh ant did it. The German cockroach did it so thoroughly that no wild population has ever been found. Several species of cellar spider, funnel weaver, and cobweb spider have done it too. What they share is a set of tolerances tuned to conditions that exist almost nowhere in nature: stable temperatures between about 15 and 25 degrees Celsius year-round, low humidity, and a reliable trickle of small flying insects drawn to indoor lights.
Outdoors, none of those conditions hold for long. A garden in July is humid and hot at noon and cool and wet by 3 a.m. A garden in January, across most of the temperate world, is lethal. House spiders, having spent millennia in cellars and thatched roofs and stone hearths, have lost the physiological armor their wild cousins still carry.
Why the garden kills them
The most immediate problem is water, though not in the direction most people would guess. House spiders desiccate quickly in dry outdoor air and drown quickly in wet outdoor air. Their cuticle, the waxy outer shell that regulates water loss, is thinner than that of their outdoor relatives. In the stable low humidity of a heated room they thrive. Drop one into a dewy flower bed and its book lungs, the strange stacked-page respiratory structures spiders use to breathe, can flood.
Temperature is the second killer. House spiders have lost most of the cold-hardening machinery their outdoor cousins rely on. Wild orb weavers produce glycerol and other cryoprotectants in autumn, essentially antifreeze in the hemolymph, which lets them survive freezing nights curled under bark. Tegenaria domestica does not bother. It has not needed to for a very long time.
Then there is competition. The garden is not empty. It is a battlefield of wolf spiders, jumping spiders, ground beetles, centipedes, birds, and parasitoid wasps, all better equipped for the terrain. A house spider dropped into that world is a tourist without a map. Its silk is tuned to catch fruit flies and clothes moths in still air, not the fast-moving prey of an outdoor understory. Its eyesight is poor. Its reflexes are calibrated for the slow ambush of a corner web, not the sprint-and-pounce economics of open ground.
The stowaway hypothesis
Molecular work on Tegenaria and its close relative Tegenaria agrestis suggests the genus originated somewhere around the Mediterranean or the Middle East, in caves and rock crevices that offered exactly the microclimate humans would later reproduce accidentally with masonry and central heating. When people started building permanent structures, the spiders moved in. When people started sailing, the spiders sailed with them. Tegenaria domestica is believed to have arrived in North America via European ships and now occupies human structures across the continent.
The same story explains why the spider you find in a Tokyo apartment looks remarkably like the one in a Buenos Aires garage. They are not convergent. They are cousins, distributed by trade routes.
What they actually eat, and why they matter
Inside a typical home, house spiders subsist on a diet of small flies, fungus gnats, silverfish, ants, moths, and occasionally each other. A single Tegenaria in a quiet corner can consume insects steadily throughout the year. Multiplied across a continent’s housing stock, the pest-control service is not trivial. The annual prey biomass consumed by the world’s spiders is substantial, and while only a fraction of that happens indoors, it lands almost entirely on insects most people would rather not share a kitchen with.
Outdoor spiders in the garden do a parallel job on aphids, thrips, mosquitoes, and cabbage moths. Extension horticulturists writing for the Bay Area gardening community have spent years trying to convince homeowners that the orb weaver stretched across the tomato plants is a free employee. The message keeps failing because the folk model conflates the two populations. The spider in the tomato plants and the spider behind the sofa are usually not the same animal and cannot swap habitats.
The autumn migration that is not a migration
Every September, newspapers run some version of the same story: spiders are invading homes as the weather cools. The truth is subtler. The spiders in the home have been there all along, usually invisible in wall voids, attics, and the undersides of furniture. What changes in September is that adult males of certain species, having reached sexual maturity, leave their retreats and wander in search of females. They cross floors in the middle of the room instead of hugging the baseboards. They fall into bathtubs, whose smooth porcelain sides they cannot climb. Suddenly they are conspicuous.
The population has not grown. The behavior has changed. And the wanderers are almost all indoor-born, indoor-raised specimens of species that would not survive a night on the lawn.
The mercy-release problem
Which brings us to the jar. The standard humane response to a spider indoors, a glass over the animal, a piece of card slid underneath, a walk to the back door, is, for a house spider, a death sentence delivered with good intentions. If a spider must be moved, it should be relocated to a garage, shed, basement, or crawl space, somewhere still enclosed, still dry, still warm enough in winter to keep the animal alive.
A house spider carried out to a hedgerow in October will likely die within days from cold or moisture. If nothing else finds it first.
How to tell the difference
The species most likely to be genuinely indoor are Tegenaria domestica and its larger relative Eratigena atrica, the giant house spider, along with Pholcus phalangioides, the pale, long-legged cellar spider that hangs upside down from ceiling corners, and Steatoda species, the false widows. All share the same tells: they build in still air, in dark corners, at the seams where wall meets ceiling or floor meets baseboard. Their webs are dusty because they are old. The spiders themselves are often pale, having spent their lives out of sunlight.
Garden spiders that occasionally wander indoors by accident, wolf spiders on a hunting circuit, orb weavers blown through an open window, look different. They are usually darker, more patterned, and they do not build webs in the house. They pace. They do not stay. Left alone, they usually find their way back out within a day, and if released outside they do fine.
A very old roommate
The relationship between humans and Tegenaria goes back further than most of the crops in the grocery store. There are spider remains in the ash layers of Bronze Age hearths. There are cellar spider egg sacs preserved in the mud-brick of Neolithic villages. Wherever archaeologists have found evidence of stable human dwellings, they have found evidence of the small brown spiders that moved in with us and never moved out.
The spider in the corner of the room right now is participating in a partnership older than writing. It eats the insects drawn to the warmth and the light and the crumbs. It asks nothing. It cannot leave. And on some evening this autumn, when a male crosses the kitchen floor in a hurry and someone reaches for a glass, the kindest thing to do is probably to let it keep going.
