Combined effects of photoperiod and temperature on the diapause of an intertidal chironomid.

Breeding experiments with a temperate zone population of the marine insect Clunio marinus (stock Helgoland-North Sea from 54° N) demonstrated an onset of larval diapause under combined short day and low temperature conditions only (examined LD 8:16 with 7 or 10°C resp.). Long photoperiods (LD 16:8) at 7-10° C or higher temperatures in shorter photoperiods completely prevented any dormancy response. The diapausing larvae were still locomotory active and fed in their benthic environment; their metamorphosis, however, stopped during a specific stage of the early imaginal disc formation in the last instar. In the experiments, the diapause was maintained up to 5.5 months and longer. In nature, the termination may be mainly controlled by temperature rises during low tide and fine weather under long day conditions, at Helgoland generally in late April or early May. The dormancy response can be classified as a larval oligopause (sensu H.J. Müller) of a potentially multivoltine species. It is supposed that this kind of diapause control is more widely distributed in aquatic chironomids.