The Simulium neavei Complex (Diptera, Simulidae) in Nyasaland

Abstract

Simulium nyasalandicum De Meillon and S. woodi De M. were originally discovered in Nyasaland but until recently scarcely anything was known about them. They belong to the S. neavei Roubaud complex the members of which are attached, in their late-larval and pupal stages, to crabs, and include at least one vector of human onchocerciasis which is a cause of blindness in various parts of Africa. I visited Nyasaland from 1 to 22 January 1960 in order to study the taxonomy (Lewis 1961) of the two species and to learn something of their habits near the type localities. Remarkably few specimens were obtained, and the scarcity of the two species raises problems which remain to be solved by future investigators. The present work is a contribution to the study of the complex in Africa as a whole rather than to the applied entomology of Nyasaland where the two species are not known to be of practical importance. Human onchocerciasis was reported from the Cholo district by Gopsill (1930) but was not regarded as an important problem by Berner & Carr (1954), and the vector may be S. damnosum Theobald which is known to be present. In order to find larvae of members of the complex, crabs were collected, usually from under stones but occasionally from aquatic vegetation, with a net 19 cm wide. Occasionally a net with a nearly semi-circular frame 38 cm wide and 21 cm high was used. Stones were turned over to expose the crabs, but some stones near Mlanje were too big to move. The traps used by Turnbull-Kemp (1960) and other types would be unlikely to catch all crabs, and a thorough method of survey is difficult to devise. The crabs were killed on the spot with a knife inserted into the ventral nerve cord, examined and put in plastic bags for subsequent detailed inspection of the branchial orifices. The work was sometimes done by a team of three, one collector wading up the left half of a stream, the other up the right, and a third person in the centre dealing with the catch and recording results. Larvae of the S. neavei complex are often found in streams where there is little or no risk from schistosomiasis, but S. woodi has been found breeding in a Nyasaland stream which flows through cultivated land and is damned during drought to store water for crops. This particular stream is not known to be dangerous but is a reminder that crab collectors should sometimes take precautions against infection, which may not be easy to devise.