The influence of water velocity on particle capture by the labral fans of larvae of Simulium bivittatum Malloch (Diptera: Simuliidae)

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Abstract
Larvae of Simulium bivittatum Malloch exposed to water velocities of 3.2–47.0 cm/s consumed a maximum of particles at velocities of 10.0–25.0 cm/s. Larvae consumed significantly fewer particles at velocities below and above the suitable (10.0–25.0 cm/s) range. The inability of larvae to capture particles efficiently at low velocities (3.2–4.5 cm/s) is attributed to the formation of thick boundary layers around filtering structures (labral fans). Steep velocity gradients in the boundary layer around fans and probably high drag forces on fans contribute to the low retention efficiency at high velocities (35.0–47.0 cm/s). The drag forces and the torque on particles at the water–fan interface enhance the adhesion of small (0.5–5.7 μm) particles, which form the bulk of food particles in the guts of larvae. The relevance of this study to the control of larvae of pest species of black flies is discussed.