The behaviour and form of Psephenus herricki (DeKay) (Coieoptera: Psephenidae) in relation to water flow*

Abstract

1. Psephenus herricki (DeKay) larvae were observed in aquaria and artificial streams using videomacroscopic techniques. Dye visualization was used to determine flow around the organisms.2. The highly flattened larvae of Psephenus herricki are able to withstand and/or avoid the force of flowing water in streams through a number of mechanisms. The legs are vital: if they cannot gain purchase the larvae can be dislodged by the lifting forces acting on the body. At the flow rates tested (up to 50 cm s−1), suction under the carapace is not needed to maintain position.3. Active pumping of water through lateral slots of the carapace and out from under the body posteriorly greatly reduces turbulence around the body and may act to decrease drag at high Reynolds numbers.4. An interpretation of the flattened body‐shape of Psephenus herrickilarvae as an adaptation to fast currents and/or to prevent the larvae from being removed from the substrate by a predator is of questionable ecological significance because individuals are usually found in crevices and on the undersides of stones.