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Investigations on the hydrochemical and physical parameters of the breeding sites of Bimulium woodi (an onchocerciasis vector) and Simulium nyasalandicum. and on their aquatic stages and associated crabs. were carried out for 13 months in the Zigi (Sigi) River system. The conductivity, pH and concentration of the main ions and silicate were all higher in the breeding sites of S. woodi than in those of S. nyasalandicum. S. moodi was restricted to small heavily shaded streams and did not enter rivers and large streams which contained higher concentrations of ammonium and nitrite. S. nyasakundicul-n was most numerous in rivers and large streams, but invaded small heavily shaded streams during the rainy season when conductivity and p H values were reduced.1. lntroduction .

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