BIONOMICS OF DROSOPHILIDAE (DIPTERA) IN HOKKAIDO. : VI. DECAYED HERBAGE FEEDERS, WITH SPECIAL REFERENCE TO THEIR REPRODUCTIVE STRATEGIES

Abstract

Ten drosophilid species breeding on decayed leaves and stems of herbaceous plants are compared for their bionomic characters : habitat preference, host plant utilization and annual life cycle. For the first two characters, which correlate with each other, the species are divided into two groups : one living in relatively dry habitats and the other in intermediate to humid habitats. From another aspect of bionomics, i.e., annual life cycle, the species are classified into two categories : univoltine species and multivoltine (mostly bivoltine) species. According to COHEN'S (1970) model for optimal timing of diapause, this phenological difference among species having similar ecological requirements is thought to result from interspecific competition for breeding resources, which are diminished in the second breeding season. The species-specific reproductive tactics are evaluated in connection with their life history characteristics. Species living in relatively dry habitats (open grasslands) allocate more energy to reproduction, i.e., make a larger reproductive effort, in order to adapt to the unpredictably fluctuating environments there. On the other hand, species differing in phenology but co-inhabiting relatively humid environments utilize different tactics on a common continuum of reproductive allocation between egg size and number. Univoltine species take the tactic of producing a large number of small eggs, while bivoltine species, the opposite one of producing a small number of large eggs.