The structure of the mouthparts and distribution of cephalic sense organs of larval simuliids was investigated using the scanning electron microscope and methylene blue staining. The effect of water velocity, temperature and quality on larval feeding behaviour was studied in an artificial stream. Short, controlled, pulses of physical and chemical stimulants were injected into the water to observe their effect on larval feeding behaviour. Too rapid for the unaided eye, movements were described frame by frame from video recordings. Food is filtered from the water by the open cephalic fans. In alternation the fans are rapidly closed , swept by the mandible to remove food particles and opened again. The frequency of this endogenous behaviour pattern was modified by environmental factors that appeared to act mainly on the interval between fan beats. The interval between fan beats was found to be inversely related to water temperature and velocity and was also affected by water borne stimulants, being significantly shorter in unfiltered natural water than particle-free distilled water. Consequently fan cleaning frequency rose as water velocity and temperature were increased and when natural food was available. Larvae responded to pulses of a wide variety of chemical compounds with bursts of mandible and maxilla movements. Fan cleaning was inhibited when these mouthpart movements occurred but filtering continued. Short pulses of inert particles at a relatively high concentration caused a similar response but when a series of pulses was delivered bursts of mouthpart movements lengthened and the fans were often closed for longer than normal, inhibiting filtering. It is suggested that overstimulation of peripheral sense organs, responding to the physical and chemical qualities of food particles, initiates the inhibition of filtering. The temporary inhibition of feeding may regulate the rate of ingestion. A simple model of larval behaviour is proposed, recognising "food gathering" (filtering) and "food ingestion" (mouthpart movements) as its main components.