The five-phase structure of annuity curves for fruit flies of the Canton-S line detected earlier has been observed at different cohort sizes, which is indicative of the stability of the feature in question. However, variability in the duration of each of the five phases and a related variability in the cohort lifespan was observed in five experimental replicates. The five-phase structure is assumed to be under rigid genetic control, whereas the certain variability in the phase duration (which determines lifespan variability) implemented in the program of fruit fly development is presumably affected by weak uncontrollable signals.