The spider fauna of Shetland is described on the basis of collections made in 1974–75 and earlier published and unpublished data. The number of species recorded is increased from 35 to 90. Work on Ronas Hill and elsewhere has shown that Shetland has a substantial array of montane spiders, many of them occurring at much lower elevations than is normal in mainland Britain: a height of 400 m on Shetland is roughly equivalent—in terms of the spider community—to one of 900 m in the Cairngorms. Shetland appears to lack, however, a number of the arctic-alpine species present at high levels in mainland Scotland and the Faroes. The spider fauna of moorland in Shetland is shown to have closely similar composition to that of a moorland area in the Pennines, although the latter has more species. Other habitats discussed include sand dunes in South Mainland and serpentine heaths in Unst.
The new information permits for the first time assessment of the spider fauna of Shetland in its zoogeographic context. Almost all of the species found in Shetland, the Faroes and Iceland are present in both Scotland and Fennoscandia; however, only two-thirds of the Faroes species and less than half of the Iceland species are also found in Shetland.
Consideration of the Pleistocene history of the area permits analysis of the origins of the faunas. Some species adapted to arctic conditions may have survived through the last glacial episode in Iceland, where there were ice-free areas, but such survival is most unlikely for the Faroes and Shetland: for Orkney the picture is unclear. There was no subsequent overland immigration to Iceland or the Faroes and little or none to Shetland, but Orkney may have received rather more of its species in this way. The rapid postglacial rise in sea level ensured that the bulk of the species in all these faunas had to colonise over water. Some species were transported by man and a few may have travelled on natural rafts, but the majority seem to have immigrated by ballooning.
Orkney probably received all its colonists from mainland Scotland and Shetland could also have done so. Immigration from Fennoscandia, however, was undoubtedly important in the case of Iceland, significant in the Faroes and possibly also contributed to the Shetland fauna. Only Iceland can be shown to have received species from Greenland.
In an Appendix the spider species recorded from Orkney, Shetland, the Faroes and Iceland are tabulated and the occurrence of these species on the Scottish mainland and in Fennoscandia and Greenland is indicated.