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The spider family Pholcidae (cellar spiders, daddylonglegs spiders) includes 1340 described species; the family is largely tropical, but several dozen species occur in North America north of Mexico, with the highest diversity in the southeastern United States (Huber 2005; Platnick 2013). In North America north of Mexico, six of the 12 genera found in this region are represented by introduced species only. Many pholcid species have adapted well to human habitats and are commonly found in corners and dark spaces in and around buildings, especially in basements. Pholcus phalangioides, for example, is common in buildings worldwide (Bradley 2013).
The cephalothorax of a pholcid is typically about as long as it is wide. The legs are extremely long and thin (this feature accounts for one of the common names for spiders in this family, "daddylonglegs spiders", although unlike daddylonglegs, which are not actually spiders, they spin webs). The legs have flexible tarsi that are usually held in a curved position. Most pholcids have eight eyes, although some have only six. When long-legged pholcids are disturbed in their web, they move rapidly, flexing their legs so that the body gyrates in a circular motion. This may make them difficult to see and perhaps difficult for their main predators, wasps, to capture (Bradley 2013). Short-legged pholcids can run rapidly (Huber 2005).
Pholcid females frequently carry their egg case in their chelicerae. The eggs are held together by a thin silken net and individual eggs are easily seen. The palps of adult males are very large and conspicuous. Web structure varies considerably within the family. Huber (2005) notes that the taxonomy of Nearctic pholcids is greatly in need of revision (this is presumably true for other regions as well). Much progress has been made, however, in understanding the higher level systematics and biogeography of the pholcids (Huber 2011; Dimitrov et al. 2013 and references therein). References cited in Dimitrov et al. (2013) provide an excellent entry into the pholcid literature.