Climatic conditions, diapause and migration in a troglophile caddisfly

1. Factors explaining the end of diapause include environmental conditions such as daily photoperiod, temperature and humidity. However, because all these factors are almost constant inside deep caves, they would hardly affect diapause termination in many animal taxa that use such habitats in which to aestivate or hibernate (such as bats and some insects, snakes or frogs). 2. An innate biological signal, based on body reserves (mass), could determine the end of diapause in cave-dwelling animals. Another possibility is that the diapausing animals may use as a stimulus convective air circulation, produced by a fall in temperature outside the cave to a value below that inside (i.e. temperature inversions). Here, we explored these two non-exclusive hypotheses explaining the end of diapause and the start of migration in the caddis Mesophylax aspersus. This species aestivates as an adult in caves, as a physiological adaptation to seasonal drought and stream drying. 3. The variation in body mass of individuals entering and leaving the cave was similar, suggesting no role for the ‘reserve level’ hypothesis in breaking diapause. However, the onset of a temperature inversion in autumn coincided with the breaking of diapause followed after a few days by the migration of the caddisflies. 4. We conclude that the seasonal air currents, produced by thermal inversions, could be the migration stimulus for caddisflies and many other animals that spend a period of diapause in caves.