Filter feeding in larvae of Simuliidae (Diptera: Culicomorpha): aspects of functional morphology and hydrodynamics

Abstract

The feeding mechanism of simuliid larvae is further elucidated by detailed study of the nature and speed of cephalic fan and mouthpart movements, and examination of water flow around larvae. Photographic observations of artificial particles flowing past Simulium vittatum Zett. larvae show the body and head to be highly streamlined and water flow to be laminar over the larva. Dissections and cinephotography show cephalic fan closure to be initiated and controlled by rotation and medial movement of the torma. As the primary fan closes, the median and secondary fans close and form raised sides to the primary fan. Sharp movements made during opening and closing of the fan suggest the presence of "click mechanisms."One cephalic fan closes and adducts every 0.231 s, and for the period analyzed, one or other fan was closed 28% of the time. It is suggested that the time fans are closed should be included in calculations of simuliid larval feeding efficiencies. Larvae with slower mouthpart movements may filter more efficiently than those with faster movements.Laminar flow through and cross-sectional shape of fan rays produce optimal flow for filtration. Probably turbulent flow at higher velocity may explain observed decreases in feeding efficiencies. Behaviour, body shape, and structure of simuliid larvae are highly adapted for optimal water flow over body and fans, even when gross flow in the habitat is turbulent.