Why Do Spiders Come Indoors in the Fall? House Spider Season

Where do the fall spiders really come from?

In Europe the spiders behind the annual “spider season” are mainly giant house spiders (Eratigena species) and the smaller domestic house spider (Tegenaria domestica). In North America, giant house spiders are common in the Pacific Northwest, while elsewhere similar roles are played by barn funnel weavers, cellar spiders and cobweb spiders.

These are synanthropic species, meaning they live alongside people. They spend their whole lives in buildings: in cellars, garages, sheds, attics and quiet corners behind furniture. A survey of 50 homes in Raleigh, North Carolina, published in 2016, found spiders in every house sampled, with cobweb spiders and cellar spiders among the most widespread. Most of the year, though, house spiders stay out of sight in their webs.

So when a large spider appears on the living room carpet in September, the most likely explanation is that it lived in the house, perhaps for a year or more, and has only now become visible.

Why do male house spiders wander in the fall?

The giant house spider grows through seven or eight molts before it becomes adult. When males make their final molt, usually in summer, their lives change. They stop hunting from a web and start searching for females, wandering from July to October and sometimes covering long distances inside a building.

Females, by contrast, stay put. They remain in their funnel webs in corners and crevices, where males find them by following silk and chemical signals. A wandering male may visit several webs.

This is why the autumn spiders are mostly:

• Males, with slim bodies, very long legs and swollen palps that look like small boxing gloves
• Active at night, when the house is quiet
• In open places, such as floors, walls and bathtubs, where they are easy to spot

Many of these males die within weeks or months after mating, which is why sightings fall off by November.

Do spiders move inside to escape the cold?

This is a widespread myth. Spiders are cold-blooded and do not need warm houses to get through winter. Outdoor species have their own strategies: many die in autumn after laying eggs, which survive the winter in silk sacs; others overwinter as young spiders or adults in leaf litter, under bark or in other sheltered spots outside.

Arachnologists such as Rod Crawford of the Burke Museum in Seattle point out that outdoor spiders rarely come into homes, and that most that do are wandering by accident. Indoor conditions are usually too dry and too short of prey for most outdoor species to thrive. Putting a house spider outside in cold weather may actually harm it, since it is adapted to life in buildings.

Why do spiders seem bigger and more common in the fall?

Late summer and fall are when many spiders reach their full size, both indoors and outdoors. Outdoors, the European garden spider and other orb weavers hatch in spring and are fully grown by August to October, spinning large webs across paths and windows. They have been there all summer, just small and easy to miss.

Weather plays a part too. Dry spells and changes in humidity can affect how active spiders are, and darker evenings mean more time with lights on indoors, which makes moving spiders easier to notice. Media stories about “invasions” also make people look for spiders more often.

Britain’s “spider season”

In the United Kingdom the autumn appearance of giant house spiders has become an annual news story, often described as “spider season” and typically peaking in September. Headlines warning of “monster spiders invading homes” are common, but the pattern is a natural part of the giant house spider’s life cycle and happens every year. There is no evidence that the spiders are getting bigger or more numerous year after year.

Are fall house spiders dangerous?

No. Giant house spiders and domestic house spiders are harmless to people. They are shy, run rather than fight, and rarely bite. Their fangs can pierce skin in exceptional cases, causing brief, local pain. For more detail, see Are house spiders dangerous?.

Why spiders end up in the bathtub

A spider in the bath has usually fallen in while wandering at night and cannot climb the smooth, sloping sides back out. It did not come up the drain, which is blocked by a water trap. Wandering males are especially likely to end up there in autumn, which is why the bath is such a common place to meet one. For other surprising spider stories, see Do you swallow spiders in your sleep?.