Abstract
Three species, Metacnephia tredecimatum Edw., Schoenbaueria annulitarsis Zett. and Simulium truncatum Lundstr. dominate the blackfly fauna of Tjulan at its outlet from lake Stora Tjultrask, Swedish Lapland. Benthic density of the larval assemblage decreases during July and there are changes in composition of the species both with time, and with distance along the river from the lake. Thus Sch. annulitarsis dominates the immediate lake-outlet, and extends into the lake, while M. tredecimatum and S. truncatum dominate in the region of the river where larvae are most numerous. Because M. tredecimatum is temporally well-separated in development from S. truncatum it contributes a decreasing percentage of the total blackfly fauna with time while the latter thus contributes a higher percentage. For all species pupation is later at increasing distance from the lake. The assemblage of species extends for c. 500 m after which typical river species begin to appear. It is proposed that the lake-outlet species withstand high population densities so that they can ingest particulate material egested by larvae upstream, this being considered to be an optimal food source.
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