This article is intended primarily to call attention to an almost entirely neglected field of entomological research, at least in this country, namely—the study of the habits and life-histories of those minute hymenopterous insects that have assumed aquatic life. In Europe considerable work has been done along this line. As early as 1836, F. Walker observed Agriotypus armatus (an anomalous Ichneumon fly) descend some distance into the water. Von Siebold (1858), W. Müller (1888), and others have shown that it is parasitic on Trichopterous larvæ. In 1863 Sir John Lubbock published his well known account of Polynema natans and Prestwichia aquatica, both with aquatic habits, the former swimming by means of its wings, the latter using its legs. Nothing was known by him regarding their earlier stages. Enock, Heymons and Willem have since reared Prestwichia aquatica from a variety of insect eggs, including Notonecta, Ranatra, Dytiscus and Pelobius.