Abstract
Monthly collections (1975-1976) from a sericostomatid caddisfly population, G. nigricula (McL.), in a northern California (USA) spring seepage indicated that age structure was numerically dominated by early larval instars (55-82% each month) but pupae (< 1-6%) were also present year-round. Severe drought in 1977 resulted in cessation of the normally permanent spring flow and total habitat loss occurred for over 3 mo., no G. nigricula apparently survived. Two years after habitat recovery (1979), a single cohort population with a clear temporal succession in age class dominance from early to late larval instars was evident. Temporally-restricted aerial recolonization of G. nigricula adults from a non-drought affected habitat most likely produced the shift to a single cohort population which has persisted through 2 generations (i.e., 2 yr).

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