Age structure of overwintered face fly populations estimated by pteridine concentrations and ovarian dynamics

Pteridines in the head capsules of face flies, Musca autumnalis DeGeer, were measured spectrofluorometrically to provide estimates of age. The flies also were dissected to determine ovarian development and fat body hypertrophy. Physiological ages in terms of degree-day accumulations were estimated among late autumn, winter, and early spring face flies. The reproductive history of eight overwintered face fly populations near Ames, Iowa, U.S.A., suggested that each had behaved as a single cohort, maturing their eggs and ovipositing according to a simple heat unit model. 83.8 +/- 16.4 degree-days above a 12 degrees threshold (DD > 12 degrees) were estimated to elapse between 1 January and the date at which 50% of overwintered cohorts had oviposited. Pteridine deposits indicated that diapausing females in late autumn had acquired a mean 96 +/- 36 DD > 9.8 degrees and the males 135 +/- 39 DD > 9.8 degrees. Soon after emergence from hibernaculae, females were average physiological ages of 135 +/- 25 DD > 9.8 degrees and the males were 152 +/- 28 DD > 9.8 degrees. Mean physiological age of overwintered females was 155 +/- 37 DD > 9.8 degrees compared with 110 +/- 38 DD > 9.8 degrees among parous flies in summer. Overwintered males in spring were an average 175 +/- 41 DD > 9.8 degrees compared with 144 +/- 65 DD > 9.8 degrees among summer flies.