Certain principles of hydrodynamics are dealt with briefly, then related to the biology of simuliid larvae (larval form and stance; feeding behaviour and vortices; microdistribution). Larval feeding can no longer be considered a passive affair. Larval anatomy, behaviour and filtering efficacy are all involved in formation of differential vortices and transport, entrainment and capture of particles. Further investigations of hydrodynamics of preferred habitats of larvae of various species should provide better information on aggregations and interrelations between larvae, their utilization of nutrients, and flow requirements.