The Yukon Territory provides a setting for its fauna of particular historical and ecological interest. Much of the Yukon was unglaciated in Pleistocene time as part of Beringia, a much larger ice-free but essentially treeless area extending through Alaska into eastern Siberia, and this whole area was cut off from the rest of North America by ice sheets. After deglaciation the Yukon was again connected to the North American continent, allowing for movements by and contacts with other faunas. The Yukon today is a distinctly northern region dominated by arctic, alpine, subarctic and boreal terrain. Nevertheless, it is relatively benign for its latitude of 60 - 69°N, and habitat diversity is enhanced by the local amelioration of temperature on south-facing slopes and in river valleys. As a result of these past and current influences, the insect fauna of the Yukon is relatively rich and distinctive, reflecting the results of evolution on a variety of scales, and comprising distinctive forest, grassland, tundra and other species. The composition of the fauna reflects the current or past prevalence of particular habitats, such as boreal forest (which supports many widely distributed North American species), shallow still waters (which support many aquatic species) and dry grasslands on warm slopes (which support many leafhoppers and heteropterans, for example). The groups reported on in this book contain about one third of the known arachnid fauna of Canada and more than half of Canada's insect fauna. In these groups, 297 species of spiders, 157 species of mites, and 2711 species of insects—or about one fifth of the Canadian species known in those groups—are recorded in the Yukon, suggesting that in total more than 6000 species of insects and 900 species of arachnids occur there. Individual species as well as different groups differ widely in ecological and distributional features according to their particular histories. However, the fauna, like the terrain, is distinctly northern; it is dominated by certain northern and widespread taxa, whereas other groups are represented by few species. The prevalence of northern groups tends to be correlated, though by no means exclusively, with their occupation of aquatic habitats (relatively favourable in the north) and with general feeding habits such as predation (relatively advantageous where specific resources are more limited). Many adaptations of structure, behaviour and life-cycle reflect the demands of cold and seasonal life zones. Overall, nearly twice as many Yukon species of insects are restricted to the Nearctic region as occur in both Nearctic and Palaearctic regions, though a few northern groups, as well as spiders and oribatid mites, have many Holarctic species. Much taxonomic evidence, such as the occurrence of sister species in the Yukon and in Asia, indicates past connections between North America and Eurasia that preceded the well known Pleistocene connection. About half of the Nearctic species of the Yukon are widespread in North America, and one third are western. These and other ranges suggest that species have come to occupy the Yukon by several different routes. For example, northern boreal ranges predominate among the Nearctic species. Therefore, many of them probably are postglacial invaders from the south and east. However, other widely distributed arctic and boreal species are known from Beringia as Pleistocene fossils, reflecting their presence there during glaciation. Several species appear to have survived the Pleistocene in both Beringian and southern refugia, because they have distinct or disjunct northern and southern populations. In several groups substantial numbers of species occur only in the (glaciated) southern parts of the Yukon and have not spread farther north; they are presumed to have entered the Yukon from the south after