Cellar spiders (family Pholcidae) are common indoors, and they definitely make webs. Their webs are not the highly-structured, symmetrical orb shapes typically associated with spiders. Instead, these arachnids construct a different kind of silk architecture suited to dark, undisturbed corners. The silk structures are loose, irregular, and three-dimensional. This architecture functions as a complex trip hazard rather than a sticky trap.
Identifying the Cellar Spider
Cellar spiders are often mistaken for the harvestman (Opiliones), leading to the shared nickname “daddy longlegs.” True cellar spiders are identified by their extremely long, thin legs supporting a small, pale, yellowish-gray body. As true spiders (Order Araneae), their body consists of two distinct segments: the cephalothorax and the abdomen. They seek dark, damp, and undisturbed environments. Their preferred habitats in human structures include cellars, basements, crawl spaces, and quiet room corners.
The Tangled Architecture of Cellar Spider Webs
The webs created by cellar spiders are known as tangle webs, or cobwebs, due to their messy and disorganized appearance. These structures are irregular, three-dimensional meshes of silk threads that lack geometric precision. Cellar spider silk is not coated with the sticky adhesive used by many web-building spiders. Instead, the chaotic, dense matrix of non-adhesive threads acts as an obstacle course that confuses and physically trips arthropods wandering into it. Spiders typically add new layers of silk over time, which is why the webs often appear dusty and neglected.
Hunting Behavior and Web Function
The web is an active component of the cellar spider’s unique hunting strategy. When prey, which can include insects like mosquitoes and flies, encounters the web, the spider rushes out and uses its long legs to rapidly shake and vibrate the entire structure. This “vibrating spider” behavior disorients the trapped victim, making it easier to manage. Once the prey is confused, the cellar spider quickly throws large amounts of silk over the struggling animal, immobilizing it before delivering a bite. The web also serves as the spider’s primary retreat and is where the female carries her loosely wrapped egg sac until the young hatch.
The cellar spider’s diet includes a wide range of arthropods, and they are known for aggressively preying on other spiders, even species much larger than themselves. They invade the webs of other spiders, such as house spiders, and even medically significant species like the black widow. The cellar spider mimics the vibrations of a trapped insect to lure the web’s owner out, ambushing the host spider once it approaches. This ability to capture larger, more formidable prey is achieved through the strategic use of silk to rapidly bind and subdue the victim, rather than relying on potent venom.
Cellar Spiders as Natural Pest Controllers
Cellar spiders pose virtually no threat to humans, as their fangs are quite short and their venom is considered relatively mild compared to other spiders. Research suggests their fangs may be too small to reliably penetrate human skin, and even if a bite were to occur, there are no documented cases of serious medical consequences. Their role as efficient predators of common household insects and other spiders makes them beneficial inhabitants of the home environment. By controlling populations of various pests, including those that are considered a nuisance or a threat, cellar spiders effectively act as natural pest controllers. Homeowners can benefit from this natural management by choosing to coexist with the spiders in undisturbed areas like basements and garages.