Locomotor Activity Patterns in Ilybius fenestratus (Fabricius) (Coleoptera, Dytiscidae) in a Central Norwegian Lake, with Evidence for Breeding-Induced Arrhythmicity

Abstract

The diel activity of Ilybius fenestratus was studied in Målsjøen, a lake in Sør-Trøndelag, Central Norway (63°14'N, 10°26'E), during 1971–1972. Adults and larvae were sampled in activity traps every week during March–October and every 2–3 weeks during November–February. The traps were emptied every 2 hrs during a 24- hr period. The diel locomotor activity of I. fenestratus was clearly nocturnal, except in the second half of July, for the males also in the first half of July, when surprisingly the population showed an arrhythmic activity pattern. The arrhythmicity is connected with the mating and egg-laying period of the species. The larvae were also nocturnal in the autumn, but showed arrhythmicity under the ice during the winter.