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this. G. caspius and G. distinctus, which have a similar but less strongly marked preference for reeds and other plants which project above water, are slightly morę gregarious but rarely or never form large schools. G. substriatus usually inhabits smali ponds and although morę gregarious than G, caspius never, in my experience, forms really large schools. The average size of a school is about twenty and aggregations of forty or fifty are not common. The individual however often swim closely togę-ther, G. marinus and G. thomsoni which are open water species inhabitating lakes and rivers, have the gregarious instinct deve-loped to a high degree, schools of several hundreds being of common occurence. G. marinus is usually to be found in lakes swimming several yards from the shore. The schools are often large and cover a great area so the individuals do not usually swim in very close proximity to their neighbours. G. ihomsoni also occurs in lakes but is morę usually found in slow flowing streams and in rivers. The schools are compact and the indi-viduals rarely wander far from them. They are generally to be found near the bank, particularly near to some bush or smali tree, and the schools rarely contain less then fifty individuals while schools of several hundred are often to be found. It is interesting to notę that the love of company shown by Gyrinus is not confined to the individuals of the same species but is extended to those of most or all other species. G. urinator will swim with G. substriatus and G. thomsoni; G. marinus with G. substriatus, G. paykulli, G. distinctus, G. thomsoni and G. caspius. When other species occur in the same locality their seems to be little or no segregation. Rare species are conseąuently to be sought for in schools of abundant forms.

For diurnal Gyrinidae the gregarious habit has an advan-tage which is unique. In generał the gregarious modę of life serves three, and sometimes four, useful purposes: the sexes are brought together, food capture is facilitated, danger is morę rea-dily perceived, and animals of noxious taste benefit by the fact that a predator having captured one does not attempt a second capture for a considerable period. The pungent and evil smel-ling Gyrinus benefits in all four ways and adds to them a fifth. In Gyrinus the gregarious habit is itself a protection as it increases the difficulty of the capture of all the individuals.