A laboratory study of fluid flow and microhabitat selection by larvae of Simulium vittatum (Diptera: Simuliidae)

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Abstract
Microhabitat selection by Simulium vittatum Zetterstedt larvae in a flume was studied at different mainstream velocities on two substrates: a thin flat plate parallel to the flow and a cylinder in cross flow. The results do not support the generally accepted assumptions that simuliid larvae keep within the boundary layer to avoid the direct influence of mainstream current and that they select the fastest velocity available when offered a longitudinal velocity gradient within their tolerance range. Instead, larvae gathered along the zone of boundary layer separation and remained along the stagnation line at the leading point of the cylinder when artificially positioned there. Further, under most conditions, larvae avoided zones of maximum surface shear stress. Larval reaction to hydraulic changes was immediate. It is hypothesized that S. vittatum larvae first scan the velocity profile at the substrate, initially moving toward increasing flow velocity (or water acceleration). They then cue on a steep velocity gradient along the body as part of the processes involved in choosing a location for suspension feeding. Such conditions would maximize particle flux through the labral fans and minimize drag forces on the bulbous posterior abdomen. This study provides the first direct evidence for microhydraulic factors as causative agents in the formation of simuliid larval assemblages.

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