Observations on the Simulium neavei complex at Amani in Tanganyika.

Abstract

The paper deals with two members of the Simulium neavi Eoub. complex found at Amani in the forest of the Usambara Mountains, Tanganyika Territory. They have already been described but are at present termed the Amani unhanded and banded forms, scientific names being witheld until the taxonomy of the whole complex is better understood.Some methods of study are described.Pupae of the unhanded form constituted a large proportion of the total number found on the common local crab, Potamon (Potamonautes) lirrangensis Eathbun, which amounted to roughly one pupa to seven crabs. This form predominated in the lower and more open parts of the drainage system examined. The adults were not seen to bite man. This and the banded form were found at a much lower altitude than any hitherto reported for the complex, probably owing to the particular nature of the local climate. Neither form was abundant.The larvae and pupae of the banded form, like the unhanded one, live externally on the crabs. The variability of wing size of the banded form is described. Observations were made on the internal anatomy of the adult with special reference to the ovaries in which the follicular relics are usually very large, indicating that the female probably bites soon after laying eggs. As in S. damnosum Theo. there is an easily recognisable class of old flies with clear Malpighian tubes. Nulliparous and parous flies have rather different biting cycles. The banded form bit man readily but only about five were taken per hour in good catching places and far fewer elsewhere.Human onchorcerciasis has been reported from Amani but is not known to do any harm there. The existence of the disease in the presence of rather small numbers of Simuliids is of interest in relation to a belief that S. neavei is a more efficient vector of onchocerciasis than is S. damnosum. Out of 359 banded females dissected 41·2 per cent, were parous and 12·8 per cent, of these were infected with nematodes, at least some of which were not Onchocerca volvulus.Taxonomy, the ovarioles and the Simuliid-crab association are discussed. The formation of the large follicular relics of the banded form is considered in relation to the history of the follicular epithelium before and after ovulation. The crab association is regarded as commensalism-, and it is suggested that the crabs benefit the Simuliids by carrying them a short distance to a suitable current and that they thus enable them to live in fast rivers and so extend their range.