Interfacial feeding behavior and particle flow patterns of Anopheles quadrimaculatus larvae (Diptera: Culicidae)

The interfacial feeding behavior, mouthpart movements, and particle flow patterns of Anopheles quadrimaculatus larvae were investigated, using videotape recordings, high-speed microcinematography, SEM, and laboratory experiments. While positioned at the water surface, larvae demonstrated 12 behaviors associated with movements of the head. In one of these, a larva rotated its head 180-degrees and directed its mouthparts against the air-water interface. The larva rapidly extended and retracted its lateral palatal brushes (LPBs) at a rate of 5 cycles/s (5 Hz), creating currents and allowing for the collection of particles. Particles moved toward the head at a velocity of 4.31 mm/s, in discrete stops and starts, as the LPBs beat. Our analyses determined that particle movement toward the mouth was governed by very low Reynolds numbers (0.002-0.009). This finding indicated that viscous forces predominated in Anopheles feeding and no inertial movement of particles occurred. According to this model, the LPBs cannot intercept particles directly, but function as paddles for particle entrainment. We did not observe the pharynx to function in particle filtration but, rather, in food bolus formation. We propose that the maxillary pilose area and midpalatal brush function as interception structures. It appeared that the LPBs do not break the surface film to feed, but collect particles from the surface microlayers. A plume of uningested particles emerged from the sides of the cibarium and descended into the water column. The plume consisted of alternately clear and dark, lenticular laminae formed beneath the larval head during the collecting-filtering feeding mode. A comparison of particle sizes from surface microlayers and gut contents of fourth instars showed that larvae ingested mainly small particles in the range of 1.5 to 4.5 mum in diameter. The potential significance of interfacial feeding by anopheline larvae in their aquatic environment is discussed.