Various aspects of the morphology of a. najas are described from a functional viewpoint, and experimental evidence is used to support some of the conclusions reached. Special attention is focused on the structure of the hypopharynx and the associated structures, and the function of the mandibular muscles is reassessed. The thorax is considered as a functional unit in relation to locomotion, and experiments on the mechanism of rowing, with its significance in regard to alary polymorphism are described. The reactions of Gerris species to surface tension are investigated. Preliminary observations are made on the function of the metasternal gland in this genus. The structure and function of the reproduotive system in both aexes are disoussed, with some observations on the eggs, oviposition and development. A new interpretation of the spermatheoa is presented. Many aspects of the morphology are shown to be adaptations to life on the surfaae film of water.