Age structure in a fly population: annual changes in the ovarian condition of a population of Musca domestica L. (Diptera: Muscidae)

Examination of ovarian morphology was used to determine the seasonal age variations in a field population of house flies. A small winter and early spring population consisted entirely of nulliparous females, mostly in the early stages (N to II) of egg development. With the advent of warm weather gonoactivity increased and newly emerged adults enlarged the population. Summer and fall numbers remained high with minor oscillations, tending upward to a peak in the fall. These populations were characterized by a large proportion of newly emerged females and a very small proportion of parous individuals, indicating a vigorously reproducing, short-lived population. The relative lack of parous females could have been due to emigration or, more likely, to death following the first oviposition. In early winter population numbers were drastically reduced with the advent of cold weather. A large proportion of the surviving females ware physiologically young (70%), and the population resembled that found at the beginning of the study. The overwintering mechanism appeared to be an adult diapause.

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