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surfacc tcnsion. These collect in the surface film which is in conseąuence richer in salts and in organie matter than is the water below, The water of the surface film is conseąuently both physically and chemically peculiar. It is neither in the state of a true solid or a true liąuid but is a thing apart.

On the under-side of the film live many smali organisms attracted by the higher proportions of the materiał wherewith to build up their bodies or by its semi-rigidity which allows them to cling to it as to a solid. Above, upon the elastic membranę, run spiders, Collembola and long-legged Hemipłera, for it is fuli of food for those above as for those below. Strongly adhesive, its grip holds smali creatures with firmness. Flying insects which fali upon the water, smali creatures which drop from bank or tree and clumsy swimmers, who break the surface, are alike trapped and struggle, helpless, until they die. Alone of all living things the Gyrinidae make their home in the surface film. They have discovered a new world: a world of the dead and the dying. With only the lower surface immersed and their weight largely supported by the elasticity of the film they occupy a position of unique physical advantage. The visco-sity of the water is fully utilized for propulsion while its fric-tion is reduced to a minimum. A boat or a bird may appear to possess the same advantage, but this is not so. They float by displacement. The Gyrinidae are supported by the surface film and so float higher and offer a smaller proportionate surface to the friction of the water, How this is achieved is obs-cure: all that we know certainly is that a part of the underside is wet and that, as HATCH¹) has shown, the surfacc film lifts the beetle so that it floats higher in the water than it would were it raised by displacement alone. Breaking the surface-film in this way the Gyrinidae may truły be said to inhabit it.

As a habitat the surface film offers certain advantages. It abounds in food and the food organisms are trapped and helpless, it offers certain physical advantages and, except for the Gyrinidae, it in uninhabited. It is also a place of great dangers. An insect which attempts to swim in the surface film has not

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1

Hatch. 1925, An Outlinc of tlić Ecology of Gyrinidae. Bul). Brooklyn Ent. Soc. Vol. XX, p. 101-114,