Seasonal change in the shape of the furca (cyclomorphosis) in H. socialis is described. During winter the fully grown juveniles have, in agreement with the description of the species, large teeth on the dens. However, in summer both juveniles and adults have much smaller teeth. The mucro becomes more heavily sclerotized in winter. Adult H. socialis fit the description of H. sahlbergi (Reuter); however, the type material of the latter differs slightly from adult H. socialis. Cyclomorphosis occurs during one moult in September-October and during another moult in May. The larger teeth in winter are suggested to be an adaptation to the jumping activity of H. socialis on the surface of the snow.