The spider family Clubionidae (sac spiders) includes around 581 described species (Platnick 2013). Around 58 of these occur in North America north of Mexico (Bradley 2013). Clubionids are wandering hunters and do not build webs to capture prey, although they do make sac-like silken retreats, often in rolled leaves, for molting and for depositing egg sacs. Clubionids are mostly medium-sized, light-colored spiders that are active at night. They occur on foliage, beneath loose bark, in leaf litter, and under rocks (Edwards 1958). The common name "sac spider" is derived from the fact that they build a compact silk retreat each morning before becoming inactive until the evening (Bradley 2013). Like most spiders, clubionids have eight eyes.
Historically, a broad range of spiders with two tarsal claws have been included in the Clubionidae. A number of families formerly treated as subfamilies within Clubionidae are now treated as distinct families and several genera formerly included in the Clubionidae are now placed in families that were always considered distinct from the Clubionidae. Some issues related to the delineation of this family remain, however, such as whether Cheiracanthium is a clubionid or, instead, belongs in the family Miturgidae (Richman and Ubick 2005 and references therein). The Nearctic clubionids were revised by Edwards (1958) although there have been a number of additions since then (see Richman and Ubick 2005 and references therein). Clubionids can be distinguished from similar-looking spiders in the family Gnaphosidae by their conical spinnerets. In clubionids, these are arranged in a compact cluster that rarely extends beyond the end of the abdomen. In contrast, gnaphosids have conspicuous cylindrical spinnerets, which are often visible from above. (Bradley 2013)
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