1. Clunio marinus is found in the eulittoral zone of the shallows around Tromsö (Norway). There is one generation per year. Clunio overwinters in the third larval stage. The reproductive season is limited to a few weeks in midsummer (June to July). 2. The imagos emerge every 12.4 h during ebb (tidal rhythm of emergence). The maximum emergence coincides exactly with the exposure of the substratum. 3. Control of emergence rhythm is not influenced by diurnal or lunar factors. No endogeneous oscillation could be shown. 4. Peaks of emergence can be induced in the laboratory by light or temperature pulses. Other factors related to the tide, such as turbulence or exposure to air, could be of (secondary) importance. 5. A rise of about 1° C or more in temperature causes an immediate peak in emergence. It also functions as zeitgeber for a peak appearing 11-13 h later (hour-glass effect). Lowering the temperature has no effect. 6. "Ligth-off" causes the immediate response, whereas "light-on" has only the programming hour-glass effect. 7. The duration of the hour-glass phase is 11 h at 20° and more than 13 h at 10°C. The Q 10 is about 1.2 (temperature independence). 8. During the hour-glass phase emergence is inhibited. 9. The mode of action of light and temperature pulses is explained with a model. A "waiting phase" is assumed for each pupa, during which emergence cannot occur without a particular stimulus. 10. The programming ensures concentration, without great susceptibility to disturbance, at the time which is biologically important for reproduction. The precisely timed emergence peak in natural conditions is caused by warming of the substratum by surface water. 11. Two different types of hour-glasses are defined and discussed. 12. The emergence phenomenon of the Tromsö population permits direct observation of an hour-glass effect. The hour-glass shows no light- or dark-dependent phases. 13. Three hypotheses on the evolution of the hour-glass are discussed. Comparison with other reports indicates that an hour-glass phase might also be present in oscillating systems.