Spiders active on snow in Southern Norway

In Norway, insects and springtails which are active on snow have been fairly well studied, but not the spider fauna. A sample of 439 spiders collected on snow during many years and from several localities in South Norway, contained 46 species, including 12 which were only identified to genus level. Spiders occurred on snow during all winter months, mainly at temperatures around or above 0 degree C, but even down to -7 degree C. Most species belonged to Linyphiidae. Comparisons with Finnish pitfall-trapping below snow and a Norwegian study on the spider fauna on spruce branches during winter show that nearly all species colonise the snow surface from the subnivean air space, probably climbing up along stems and bushes penetrating the snow. In late winter, the spider fauna on snow is dominated by Bolepthyphantes index, which constructs nets in small depressions in the snow surface, for instance footprints of animals, and catches winter active springtails. During very warm days in April, subadults of Philodromus sp. can start wind dispersal by sending out "flying threads" from trees and may land in large numbers on snow. From there they probably seek snow-free patches. As a group, spiders evidently contain many cold-adapted species, and Finnish studies have showed that almost any species may be encountered on snow. Long-term pitfall trapping near Bergen showed that some spider species had their main activity during winter, indicating winter reproduction. For most spiders, their occurrence on snow in mild weather may simply be a continuation of their normal subnivean activity. There should be a large evolutionary potential for more spider species to adapt to use the food resource represented by winter active insects and springtails on snow.