Overwintering aggregations of Leiobunum paessleri in caves and mines (Arachnida, Opiliones).

Abstract

In the forested mountain areas of southwestern Canada, Leiobunull1 paessleri (Roewer) form overwintering aggregations in the twilight regions of moist caves and mines. Each aggregation consists of adults of both sexes and may contain as many as one or two thousand individuals. The opilionids congregate from August to October and disperse from April to May. Colder temperatures seem to increase the density of the aggregations. When dense aggregations are formed (up to 2.6 individuals per em'), the opilionids hang by their pedipalps with their legs extended straight down from their bodies. Explanations of the aggregation phenomenon include: optimal temperatures and humidities, reduced desiccation, heat production, increased mating success, and reduced predation. Overwintering mortality factors potentially include cold, desiccation, starvation, predation and disease.