Cellar Spiders: The Harmless Web-Spinners in Your Basement

Cellar spiders are among the most common indoor spiders worldwide. Often called daddy longlegs (though they share this nickname with harvestmen, which are not spiders), cellar spiders are fragile, long-legged arachnids that thrive in the dark, damp corners of our homes.

Identification

Cellar spiders belong to the family Pholcidae, which includes about 80 species in North America.

• Size: Body length of 2 to 10 mm, but leg span can reach 50 mm or more.
• Body shape: Small, oval body with extremely long, thin legs.
• Color: Pale gray, tan, or yellowish-brown. Nearly translucent in some species.
• Eyes: Eight eyes arranged in two groups of three with two eyes between them, though patterns vary by species.
• Web: Loose, irregular, tangled webs built in corners and ceiling junctions. Webs accumulate over time, creating the "cobwebs" homeowners find unsightly.

Common Species

• Long-bodied cellar spider (Pholcus phalangioides): The most common species indoors, with an elongated body and very long legs.
• Short-bodied cellar spider (Spermophora senoculata): Smaller, with a round body and proportionally shorter legs.

The Daddy Longlegs Myth

A persistent myth claims that cellar spiders have the most potent venom of any spider but fangs too small to bite humans. This is false on both counts. Cellar spider venom has been studied and is not particularly potent. Their fangs can technically penetrate human skin, but they very rarely bite, and any bite would produce, at most, a mild, brief burning sensation.

Cellar spiders are not dangerous in any way.

Habitat and Behavior

Cellar spiders prefer dark, humid environments. Indoors, you will find them in:

• Basements and crawl spaces
• Garages and sheds
• Bathrooms, especially under sinks
• Closets and undisturbed corners
• Behind furniture and appliances

Web Vibration Behavior

When disturbed, cellar spiders perform a distinctive defensive behavior: they vibrate rapidly in their webs, becoming a blur. This behavior is thought to confuse predators by making the spider harder to see and target.

Predatory Behavior

Cellar spiders are effective predators of other spiders, including potentially dangerous species. They have been documented killing black widows and other spiders that wander into their webs. They use their long legs to wrap prey in silk from a safe distance.

Should You Remove Cellar Spiders?

Cellar spiders are beneficial — they consume mosquitoes, flies, gnats, and other household pests. However, their webs can become unsightly, especially when they accumulate dust and debris.

Management Options

• Tolerate them: In basements and crawl spaces, cellar spiders provide free pest control. If they are out of sight, consider leaving them alone.
• Web removal: Regularly sweep or vacuum webs to manage the cosmetic issue while keeping the spiders themselves.
• Reduce humidity: Since cellar spiders prefer damp conditions, running a dehumidifier in your basement can make the environment less hospitable.
• Seal entry points: Caulk cracks and gaps to reduce the number of prey insects entering your home, which in turn reduces the spider population.

When to Take Action

Chemical treatments are rarely warranted for cellar spiders. If you want to reduce their numbers:

• Remove webs and egg sacs with a vacuum.
• Place sticky traps in corners.
• Address moisture issues that create favorable habitat.
• Follow our spider prevention tips.

Cellar Spiders and Black Widows

One of the most interesting behaviors of cellar spiders is their ability to prey on much more dangerous spiders. Cellar spiders have been documented catching and killing black widow spiders that wander into their webs. They use their long legs to throw silk at the black widow from a safe distance, gradually entangling it before delivering a bite.

This predatory behavior has led some pest control professionals to recommend leaving cellar spiders in place, particularly in regions where black widows are common. Having cellar spiders in your basement or garage may actually reduce the risk of encountering more dangerous species.

Reproduction and Population Growth

Cellar spiders reproduce readily indoors:

• Females carry their egg sacs in their jaws, loosely wrapped in silk, rather than attaching them to the web.
• Each sac contains 20 to 60 eggs.
• Females can produce multiple sacs per year.
• Baby spiders often remain near the mother's web initially before dispersing.

This reproductive capacity means cellar spider populations can grow significantly if left unchecked, leading to an accumulation of webs that many homeowners find unacceptable.

For more on common indoor spiders, see house spiders and our guide on types of spiders.

Expert Insights

Cellar spiders are one of the most common indoor spiders I encounter in my IPM work. I have lost count of the number of times I have been called about 'daddy longlegs infestations' only to find perfectly harmless cellar spiders. I always take the opportunity to educate homeowners that these spiders are actually excellent pest predators — I have watched cellar spiders capture and consume black widows in garages, which makes them surprisingly useful allies. — Sarah Mitchell, BCE

Sources and References

• University of California Riverside Spider Research
• Burke Museum of Natural History and Culture
• National Pest Management Association (NPMA)

Main Causes

Cellar spiders (family Pholcidae) establish indoors because the conditions match their requirements: low light, elevated humidity, limited disturbance, and reliable prey insects. Basements and crawl spaces are primary habitat because moisture levels are naturally higher and human traffic is low, allowing webs to persist undisturbed. Garages offer similar conditions with additional insect traffic through gaps under doors. Bathrooms provide humidity from regular water use. The prey that sustains indoor cellar spider populations includes mosquitoes, gnats, small flies, and even other spiders. Any home with a moisture problem - condensation on cold pipes, a leaking foundation, or poor basement ventilation - will have above-average cellar spider activity because both the spiders and their prey thrive in damp conditions.

Risk and Severity

Cellar spiders present no medical risk to humans. The persistent myth that they carry the world's most potent venom is false - no scientific analysis has found their venom to be particularly toxic, and the claim about fang size being too small to bite has also been debunked. They can technically bite, but any resulting symptom is a brief, mild burning sensation that resolves in minutes. The NPMA classifies them as a nuisance pest only, not a health concern. Their primary practical impact is cosmetic: their loose, accumulating webs create the classic "cobweb" appearance in undisturbed areas. In basements shared with black widows, cellar spiders may actually reduce widow populations through predation.

Prevention

Reducing humidity is the most effective cellar spider prevention measure. Run a dehumidifier in the basement targeting below 50 percent relative humidity. Improve cross-ventilation in crawl spaces by ensuring vents are unobstructed. Fix any leaking pipes, foundation seeps, or condensation issues that elevate moisture. Remove webs regularly with a vacuum - this eliminates egg sacs and discourages re-establishment at the same site. Seal cracks in the foundation and around basement window frames to reduce both spider and prey insect entry. Sticky traps placed in corners capture wandering individuals and provide a monitoring baseline. These steps, combined with regular cleaning, will reduce cellar spider populations without chemical treatment.

How to Identify

Identification matters because risk and control differ significantly by species. Most household spiders — cellar spiders, common house spiders, jumping spiders, wolf spiders — are harmless and beneficial. Two species in North America warrant caution: the black widow with its shiny black abdomen and red hourglass marking, and the brown recluse with its violin-shaped marking and uniform tan-brown coloring without leg banding. Check webs for shape and structure: tangled cobwebs in corners indicate cellar or common house spiders; funnel-shaped webs near ground level indicate funnel-web species; sheet webs across grass are usually grass spiders. Single sightings without webs are usually transient outdoor species and do not indicate an infestation.

Solutions and Actions

For most spider species the goal is removing webs and reducing prey rather than chemical treatment. Vacuum or sweep down all visible webs weekly, including egg sacs, in garages, basements, attics, eaves, and exterior corners. Reduce indoor insect populations by maintaining screens, sealing entry points, and addressing any active pest issue — fewer insects means fewer spiders. Apply a residual insecticide barrier to the foundation perimeter, around windows and doors, and in eaves to deter newly arriving spiders. For confirmed black widow or brown recluse populations in storage areas, use professional pest control, wear long sleeves and gloves when handling stored items, and shake out shoes and clothing left in garages or basements. Single sightings indoors without webs are usually transient and need no chemical response.

Frequently Asked Questions

Are cellar spiders the same as daddy longlegs?

Cellar spiders are often called daddy longlegs because of their extremely long, thin legs, but they should not be confused with harvestmen (Opiliones), which are not spiders at all. True cellar spiders (family Pholcidae) are spiders that build webs, while harvestmen do not produce silk and have a single-segment body.

Is it true that cellar spiders have the most potent venom?

No. This is a persistent myth with no scientific basis. Cellar spider venom has never been shown to be particularly potent, and their small fangs can technically penetrate human skin but produce only a very mild, brief burning sensation. They are completely harmless to humans.

Should I get rid of cellar spiders?

Cellar spiders are harmless and can actually help control other pests, including other spiders. If their messy webs bother you, simply remove the webs with a broom or vacuum. Reducing moisture and improving ventilation in basements and crawl spaces can help reduce their numbers.

What should I recheck first for cellar spiders?

Recheck the exact place, timing, and repeated signs connected with cellar spiders before changing your plan. A single sighting or old web can mean something very different from fresh activity in several rooms. Confirm whether insects, clutter, moisture, gaps, or stored items are supporting the issue, then match the response to what you actually found.