The Daddy long-legs spider, also called the cellar spider, vibrating spider, or house spider, is a true spider and not a harvestman. Daddy longlegs is a name that is used for several unrelated arthropods with extremely long and thin legs, including these spiders, the harvestmen and tipulid crane flies. The Daddy long-legs spiders comprise the Pholcidae family, in the suborder Araneomorpha .

These spiders are web weavers. The spider hangs inverted in a messy, irregular, tangled web. These webs are constructed in dark recesses, in caves, under rocks and loose bark, abandoned mammal burrows and undisturbed areas in buildings and cellars, hence the common name, "cellar spiders". The web has no adhesive properties but the irregular structure traps insects, making escape difficult. The spider quickly envelops its prey with silk and then inflicts the fatal bite. The prey may be eaten immediately or stored for later. When off their webs, pholcids walk with an unsteady, bobbing action.

When the spider is threatened by a touch to the web or when too large a prey hits the web, the spider becomes invisible by vibrating rapidly and becoming blurred, and for this reason have sometimes been called "vibrating spiders". Doing so might make it difficult for a predator to see exactly where the spider is, or may increase the chances of capturing insects that have just brushed their web and are still hovering nearby.

Certain species of these seemingly benign spiders invade webs of other spiders and eat the host, the eggs or the prey. In some cases the spider vibrates the web of other spiders, mimicking the prey to lure the host of the web closer.

Pholcids are fragile spiders, the body being 2 to 10 mm in length and the legs are up to 30 mm long. Pholcus and Smeringopus have cylindrical abdomens and the eyes are arranged in two lateral groups of three and two smaller median contiguous (together) eyes. Spermaphora has a small globose (round) abdomen and its eyes are arranged in two groups of three and no median eyes. Pholcids are grey to brown with banding or chevron markings.

There is an urban legend stating that this family of spider is extremely venomous to humans. These spiders are harmless to man as their jaws are unable to penetrate human skin and the venom dose is also too minute (see University of California Riverside Spider Myths Page). In 2001/2, Discovery Channel's Mythbusters set out to test this myth and, on mice tests, Black widow venom was significantly more toxic. One of the show's hosts was bitten, and the bite produced little more than a mild shortlived burning sensation.