The aquatic stages of the cattle biting pest, Simulium chutteri Lewis utilize river flow in their dispersal and colonization behaviour. Peaks of drifting activity in S . chutteri larvae occurred in the early morning and late afternoon. It is deduced that female flies scatter eggs onto slow flowing waters upstream of rapids . Larvae hatch from drifting eggs and colonize substrates in slower flowing regions upstream of rapids, while later stage instars move into faster flowing regions within rapids where they complete their development . This microhabitat selection by S. chutteri leads to rapid attainment of large population sizes in suitable sections of river and reduces competition between different stage larvae . In their utilization of a variety of microhabitats the larvae of this species differ from co-existing simuliid species which restrict developmental stages to single habitats .Catastrophic drift was recorded for S . chutteri larvae and could be a mechanism to regulate population size.Drift of simuliid larvae off rapids was not related to benthic population densities in the rapids and was therefore not due to excessive production . It is concluded that larval drift off rapids is related to habitat disturbances associated with water flow fluctuations and the activity of aquatic predators and other animals .The distribution of S. chutteri in the Vaal River is restricted by oviposition requirements of the adult female. Knowledge of drift behaviour and water flow requirements of Simuliidae have been applied to manage the population size of S. chutteri in the Vaal River. Hydrobiologia 133, 143-154 (1986) .

The history of the invasion of 42 catchment regions in southern Africa (excluding Mozambique) by an altricinl species (Cyprinus carpio), a moderately precocial s p i e s (Micropferur saltmides) and two highly precocial species (Omochromis macrochir and 0. mossambicus) is examined. The abundance of these species in different catchments is discussed in relation to the diversity of indigenous species. Results suggest that: a. Dcpauperate catchments of the southern C a p are vulnerable to invasion irrespective of the life-history style of the alien species, compand to catchments (in Zimbabwe and north eastern Transvaal) with a higher diversity of indigenous species.Prccocial species are more successful than altricial species in establishing populations in natural waters in more diverse ecosystems.Degradation and modification of the environment (particularly the buiiding of impoundmmts) has played an important rob in increasing the susceptibility of freshwater ecosystems to invasion.b.c.

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