Life Cycles of Two Species of Stenopsyche (Trichoptera: Stenopsychidae) in Tohoku District

The life cycles of the two net-spinning caddisflies, Stenopsyche marmorata and S. sauteri, were compared in the Mamigasaki River in Yamagata, Tohoku District. S. marmorata had a slow-growing overwintering generation pupating from May to June with a long flight period, and a fast-growing summer generation with pupating in August with a short flight period. The mean individual weight of the overwintering generation was heavier than that of the summer generation in fifth instar larvae and adults, probably because pupation and emergence of the overwintering generation were suppressed by low spring water temperature, although larval growth continued. S. sauteri was a clearly univoltine life cycle. The development of S. sauteri was overtaken by that of S. marmorata in May when S. sauteri did not begin to pupate, although water temperature was high enough. S. sauteri began to pupate in July when the overwintering generation of S. marmorata finished pupation. We speculated that the staggered life cycle might facilitate the coexistence of the two species from July to September.