Cellar spiders originated in the tropical regions of Southeast Asia roughly 50 million years ago, but the ones in your basement almost certainly came from somewhere much closer. These spiders have spread to every continent except Antarctica by hitching rides with human activity and thriving in the stable, sheltered environments that buildings provide. Understanding where they come from, both evolutionarily and practically, helps explain why they’re so common indoors and so hard to keep out.
Their Ancient Origins in Southeast Asia
The cellar spider family, Pholcidae, traces back to the Early Eocene epoch, between 54 and 43 million years ago. Research published in Zoological Research pinpoints their most recent common ancestor to the region of northern Indochina and southeastern Central Asia, in what scientists call the eastern Neo-Tethyan region. This was a period of dramatic geological change: the Indian tectonic plate was colliding with Asia, reshaping coastlines and mountain ranges. Those shifts in terrain likely drove the first splits in the cellar spider family tree, pushing populations into separate habitats where they evolved independently.
From that tropical starting point, cellar spiders diversified into over 1,800 known species. Their original habitat was warm, humid, and dimly lit, think forest floors and cave entrances. That preference for dark, stable environments is exactly what makes modern buildings so attractive to them.
How They Get Into Your Home
Cellar spiders don’t migrate indoors in dramatic waves. They enter through small cracks in foundations, gaps around windows and doors, openings where pipes or wires pass through walls, and unsealed vents. Some arrive as tiny spiderlings carried on air currents, a dispersal method called ballooning. Others simply walk in through any ground-level opening, especially during cooler months when outdoor conditions become less hospitable.
Once inside, they rarely leave. Your home offers everything a cellar spider needs: consistent warmth, low light, and a steady supply of small insects. Basements, crawl spaces, garages, closets, and ceiling corners are prime real estate. They’re particularly drawn to areas with some moisture, which is why bathrooms and laundry rooms are common spots. Unlike many outdoor spiders that wander indoors by accident, cellar spiders are genuinely adapted to indoor life. The most common household species, Pholcus phalangioides (the long-bodied cellar spider), is now considered cosmopolitan, found throughout the world in human structures. In many temperate climates, it can no longer survive outdoors year-round and depends entirely on buildings for shelter.
Why They Spread So Successfully
Cellar spiders owe their global reach to human commerce and migration. As people moved goods by ship, rail, and truck over centuries, cellar spiders tagged along in crates, furniture, and building materials. Their tolerance for low food availability and their ability to survive in undisturbed corners made them ideal stowaways. Today they’re established on every inhabited continent.
Their reproductive strategy helps too. A female long-bodied cellar spider produces about three egg sacs over her lifetime, each containing between 13 and 60 eggs. The short-bodied cellar spider (the other common household species) produces 10 to 27 eggs per sac. Development from egg to adult takes about a year, and adults can live an additional two years. That means a single female in your basement can produce well over a hundred offspring during her life, all without ever stepping outside.
What They Eat (and Why That Matters)
Cellar spiders are effective predators that feed on insects, other spiders, and small invertebrates. Ants are particularly common prey. But what makes cellar spiders unusual is their aggression toward other spiders. They routinely invade the webs of other species, consuming both the web’s owner and whatever prey was caught in it. They’ve been observed hunting around the edges of other spiders’ webs, picking off males that approach to mate.
This predatory behavior is one reason cellar spiders often become the dominant spider in a home over time. If you notice fewer other spider species but more cellar spiders, that’s not a coincidence. They’re actively displacing the competition.
The Vibrating Defense Trick
If you’ve ever touched a cellar spider’s web and watched the spider blur into a rapid vibration, you’ve seen their signature defense mechanism. When disturbed, cellar spiders gyrate their bodies so quickly in their webs that they become nearly invisible, appearing as a smudged blur rather than a recognizable spider shape. This behavior is thought to confuse predators that rely on visual targeting. A mother carrying her egg sac will do the same thing, protecting both herself and her offspring.
Can They Actually Bite You?
There’s a persistent myth that cellar spiders have the deadliest venom of any spider but can’t bite through human skin. Neither part is true. Research on pholcid spider venom published in Frontiers in Ecology and Evolution confirmed that adult cellar spiders can pierce uncalloused human skin with their fangs. The few documented bites on humans resulted in a mild sting with no lasting effects. Their venom contains an interesting mix of proteins and peptides, but a single spider’s venom is not dangerous to humans or other mammals. They’re essentially harmless.
Reducing Cellar Spiders at Home
Because cellar spiders depend on the conditions your home provides, the most effective approach targets those conditions directly. Reducing humidity with a dehumidifier, especially in basements and crawl spaces, makes areas less appealing. Sealing cracks in your foundation, fitting door sweeps, and repairing torn window screens cuts off entry points. Regular vacuuming of corners, ceiling joints, and behind furniture removes both spiders and their messy, irregular webs. Since cellar spiders eat other insects, reducing your home’s overall bug population by keeping food sealed and trash managed also limits their food supply.
That said, cellar spiders are among the most benign housemates you could have. They don’t damage structures, they don’t bite unless physically pressed against skin, and they actively reduce populations of other insects and spiders. For many people, the thin, tangled webs in a basement corner are a small price for free pest control.