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Drift collections of pupal exuviae from 2nd-5th order streams of the Guanacaste National Park in Northwest Costa Rica and from 3rd-6th order streams of the West African countries of Guinea, Ivory Coast, Sierra Leone and Togo have revealed high community (alpha) and regional (beta) levels of species diversity.Samples have been processed from 13 streams of the Guanacaste N. P. including two, Quebrada Las Yeguitas (2 stations) and Rio Tempisquito, for which year-long series were available. A total of 266 species level taxa have been recognized from these streams and are distributed over the higher taxa as follows: Chironomini-73, Pseudochironomini-4, Tanytarsini-55, Orthocladiinae-92, Macropelopiini-5 and Pentaneurini-37. The two sites on the Q. Las Yeguitas have a combined total of 216 species, and together with R. Tempisquito a total of 238 species. The single richest sample (Q. Las Yeguitas, Orosi station, 1990-Jan.-9) contained 102 species.Samples have been processed from 31 West African streams which, for the most part, are represented by single samples. A total of 299 species level taxa have been recognized from these streams which are distributed over the higher taxa as follows: Chironomini-139, Tanytarsini-62, Orthocladiinae-56, Coelotanypodini-2, Macropelopiini-3 and Pentaneurini-37. The single largest collection, containing 175 species, was taken from the Upper Dion River, a 6th order stream in the Upper Niger basin of the Guinean highlands.The compositions of the two regional faunas show similarities in two general features: they are both taxonomically narrow (few subfamilies and tribes) and taxonomically deep (many closely related genera, often with a relatively large number of closely related species). The high alpha diversity values require an ecological explanation, most probably along the axes of resources (time, space and food). The high regional diversities require an historical explanation. It is proposed that great climatic variation during the Pleistocene coupled with great geographic alteration of the connection between North and South America may be responsible for both the taxonomic narrowness and depth seen in the chironomid faunas of West Africa and Costa Rica.

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