Of the 13,800 species distributed in the Holarctic Region only 395 are known both from Eurasia and North America. Of these only 105 species are distributed throughout the whole Holarctic (circum-Holarctic species). In addition, 28 species have an almost complete Holarctic distribution, occurring from Europe to northwestern North America (subcircum-Holarctic species). Species with a circum-Holarctic distribution were found in 13 families. The highest numbers of circum-Holarctic species were in the families Linyphiidae (37), Theridiidae (14), Araneidae (13) and Gnaphosidae (11). The percentage of the circum-Holarctic species among the Holarctic spiders is highest in Philodromidae (2.4%), Araneidae (2.2%), Theridiidae (2.0%) and Tetragnathidae (1.9%). These families encompass mainly herb-bush-tree dwellers. Somewhat unexpectedly it was found that most circum-Holarctic species occupy the boreo-nemoral zone (41%), or may even have a polyzonal range (23%). Twenty-nine species (28%) of the circum-Holarctic spiders have a northern distribution; most of them occurring both in arctic and boreal zones.
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