The developmental time of Schistosoma haematobium in Bulinus truncatus snails (field strains) was determined in the laboratory at different constant temperatures between 18 and 32 degrees C. The basic relationship between the length of the minimum prepatent period (y, in days) and the temperature (x, in degree C) is given by the hyperbolic formula y = 295/(x-15.3), 15.3 being the theoretical "developmental null point" and 295 the constant time-temperature product. The shortest prepatency was 17-19 days at 30, 31 and 32 degrees C; at 18 degrees C, cercarial development required at least 106-113 days. The maturation time frequently exceeded the possible minimum by several weeks. No schistosome matured in our experiments at 17 degrees or 33 degrees C. The cercarial release per snail at weekly exposures showed a maximum at 25 degrees C with a geometric mean of 109 cercariae (95% confidence limits 79-149), decreasing to 8 (2-30) at 18 degrees C and 62 (38-100) at 32 degrees C. The absolute maximum of cercariae shed by one snail during 5 h "stimulation" was 2,150 in a 25 degrees C batch, 48 at 18 degrees C and 529 at 32 degrees C. The epidemiological application, the prognosis of the transmission period and the estimation of the transmission potential in relation to climatic conditions are discussed.

Laboratory experiments have permitted the quantification of the developmental times (prepatent periods) of Schistosoma mansoni in the snail over the whole possible range of constant temperatures. The basis relationship is satisfactorily described by a hyperbola of the formula y = 268/(x-14.2), y being the minimum time from miracidial infection to cercariae shedding (in days) x the mean temperature, and 14.2 the theoretical temperature threshold (in degrees C). Cercariae production takes place within the limits of +16 degrees C and 35 degrees C, the number of cercariae being low and the mortality of snails high at the extreme values. Long-term alternations between two temperature levels resulted in prepatent periods corresponding exactly to the proportional time-temperature products. However, slight accelerations of up to 7% and more could be observed when the prepatency began in a period of high temperature. The number of cercariae each snail shed during a period of one hour exposure per week decreased from about 1,500 at 18 degrees C to about 250 at 16 degrees C. Shedding ceased completely after 1-2 weeks at 15 degrees C.

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The arctic population of the intertidal midge Clunio marinus (location: Tromsö, Norway) shows a tidal rhythm of emergence (period: 12.4 hours) in midsummer. The emergence time exactly coincides with the initial exposure of the habitat during the ebb.When the animals were bred in a 24 hour light-dark cycle involving 16 hours of light, emergence occurred mainly 10-11 hours after light-on. When the animals were subsequently placed under constant light, no rhythm could be detected. When constant light was interrupted by a single period of 6 hours of darkness, only one peak of emergence was evoked, again 10-11 hours after light-on. Similar results were observed when in constant darkness a single period of 6 hours of lower temperature was offered.The mechanism of control differs from that of a southern population (location: St. Jean-de-Luz, France) which has a circadian clock mechanism and shows a free-running rhythm of emergence in constant light. It is postulated that the tidal rhythm of the Tromsö population is controlled by an hour-glass mechanism which starts its run of at least 10-11 hours during the preceding ebb.