1. New methods for the observation of moulting-, pupation-, and emergence-rhythms under natural and experimental conditions are described. 2. The larvae feed during all hours of the day. Rhythmic changes in their behaviour were not observed. 3. The drift rate of medium and large larvae is very low, if the animals are not disturbed (0.05% per day, n=5,500). The drift rate of young larvae seems to be much higher, but could not be determined quantitativly. Drift rhythms must not be concerned with circadian rhythms in Simuliidae. 4. The larvae moult and pupate during all hours of the day. The number of moults and pupations reaches its maximum in the afternoon. 5. The duration of the pupal stage depends on temperature and the hour of pupation. In the stream Kaltisjokk it lasts about 6-7 days. 6. The seasonal emergence of 9 species is compared in two years. 7. All species (10) emerge during the daylight hours. The rhythm persists in the continous light of the subarctic summer and under artificial LL and DD. 8. Emergence is timed by changes in light and temperature. Within ecological limits light is the dominant Zeitgeber. 9. Under LL and DD the Simuliidae emerge two hours after a rise in temperature. 10. Under natural illumination and low temperatures the emergence is concentrated round noon. In relatively high temperatures the distribution becomes broader and sometimes biphasic. There were no specific differences between species and sexes. 11. Simuliidae fly mainly in the morning and evening to a CO2-trap. Helodon ferrugineus was caught in light traps during the night. © 1969 Springer-Verlag.
