Historical distributions in Britain of two species of large house spiders, Tegenaria saeva and T. gigantea (Araneae, Agelenidae), and their evolutionary implications

Current relative distributions and evolutionary inter-actions of two species of large house spiders. Tegenaria saeva and T. gigantea. vary geographically in Britain. To understand fully their underlying causes requires knowledge of species ranges in the past. Here. information from five sources: (a) historical distribution maps based on county and vice county lists; (b) a re-examination of museum and other collections; (c) catalogues of museum specimens recently re-examined by experts; (d) scrutiny of the literature; (e) a request published in a Wildlife Trust magazine, is used to determine historical distribution patterns. It is shown that the northern limits of both species have generally increased through time and that this phenomenon is not a result of variable recorder effort. In the south of England. the species boundary in Dorset has apparently been stable for at least a century. The ecological and evolutionary implications of these results are discussed.