GEOGRAPHICAL VARIATION IN THE LARVAL DIAPAUSE OF CHYMOMYZA-COSTATA (DIPTERA, DROSOPHILIDAE)

Induction of the larval diapause in Chymomyza costata is controlled by two mechanisms, temperature-dependent induction and a photoperiodic r esponse. Geographical, clinal variation was observed in both mechanism s. Critical photoperiods increased northwards 1 h 9 min per 5 degrees of latitude. The dine relaxes north of 65 degrees N, however. The crit ical temperature measured under long-day conditions varied little in a reas south of 63 degrees N, but increased from 13 degrees C at that la titude to 17 degrees C at 65 degrees N and to 24 degrees C at 68 degre es N. Selection experiments revealed extensive intrapopulation variati on in both diapause characteristics. The mean temperature In the middl e of the breeding season for C. costata is above the long-day critical temperature in areas south of 63 degrees N, the same or slightly lowe r than the critical temperature in areas between 63 degrees N and 66 d egrees N and well below the critical temperature in areas north of 67 degrees N. The relative importance of temperature and photoperiod in d iapause induction changes from south to north, the photoperiod being t he main determinant in the south and temperature assuming the main rol e in the north. The forms and positions of the two dines can be explai ned when data on the estimated number of generations per year are comb ined with data on weather and illumination conditions at each locality .