The velocity preferences of larval and pupal blackflies were studied experimentally by comparing the colonization of plastic strings placed in different velocity ranges; and also by investigating the simuliid microdistribution under natural conditions in the river. This required a novel method described for taking spot measurements of water velocity. 2. Simulium mcmahoni de Meillon, S. hirsutum Pomeroy and S. cervicornutum Pomeroy were most abundant in slow velocities (0.3-1.0 m s-1), S. colasbelcouri Grenier & Ovazza and S. hargreavesi Gibbins preferred 1.1-1.8 m s-1, S. squamosum Enderlein and S. vorax Pomeroy 1.9-2.2 m s-1. 3. The larvae of S. hargreavesi and S. cervicornutum preferred a higher velocity range compared with their pupae. 4. In moderate velocities (1.1-1.4 m s-1), the abundance of S. hargreavesi was greater under turbulent conditions than in smooth-flowing water, but this was reversed at very high velocities (2.3-2.6 m s-1). 5. Velocity had no apparent effect on substrate preference when substrates of different flexibilities were compared for three blackfly species. All avoided the most flexible substrates and preferred ones consisting of rigid articulating plates. Anisopteran predators were found to have a similar substrate choice.
