REASONS FOR THE SHIFT IN DOMINANCE BETWEEN SIMULIUM (N) VERNUM AND SIMULIUM (S) ORNATUM (DIPTERA, SIMULIIDAE) ALONG THE CONTINUUM OF AN UNPOLLUTED LOWLAND STREAM

From a little lowland stream, parallel semiquantitative samples of lar vae and inspection and measuring of egg masses over a period of three years are presented. There is a shift in dominance between Simulium ve rnum and Simulium ornatum along a stretch of only 100 m, but scarcely any change in water quality in the first 100 m downstream of the woods . The differences between the distribution of these two species are du e to the oviposition habitats selected by the females: The reason is a n abrupt change from shaded woodland section to open grassland.