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four strong curved hooks. The larva has the habit of rearing up the head and first thoracic segment until the head stands almost at right angles to the body. This habit, associated, as it is, with a flattened head and dorsally directed mandibles, sug-gests that the insect normally seizes its prey from beneath. The larvae of Dineułus have been observed to climb amongst weeds and it is generally supposed that the larvae of Gyrinus spend their lives in a similar habitat. It is possible that they occasionally do so, but I have found them crawling on the bottom and burrowing in the soft mud and I believe this to be their normal modę of life. The slender legs, each armed with a pair of claws, and the abdominal hooks suggest that they are walkers and climbers so that the burrowing habit is, no doubt, one recently acąuired,

Pupation takes place on land in a chamber built of earth and sand grains etc. The belief that they make a papery cocoon on emergent vegetation is mistaken, This story orginated with MODEER in 1770 and has been generally accepted until recently, RICHARD Bott¹) has now settled the matter quite defi-nitely, and there can be no doubt that Gyrinus pupates in the usual manner of Dyłiscidae and of other genera of Gyrinidae. The larval skin is cast three days after the formation of the pupal chamber; seven days later the imago emerges.

The adult insect is a remarkably compact and specialized type and its habitat is unique. The surface of the water appears to be covered with a thin elastic membranę. This is not entirely an illusion, That powerful force which holds together the mole-cule of water, and which has been estimated to be equivalcnt to a pressure of eleven thousand atmospheres, is at the surface, in part, unsatisfied: a surface tension thus arises. Above a temperaturę of 4°C water consists of morę than one kind of mole-cule but as these molecules which reduce the tension congre-gate at the surface, a layer there exists which differs some-what from the underlying water. This is not all. Natural waters contain substances in solution most of which tend to decrease

*) B o 11 H, R,, 1928, Beitrage zur Kenntnis von Gyrinus natator substria-tus Steph. I. Lebensweise und Entwicklung. II, Der Sehapparał, Zs, f, Morph.

Okol, d, Tiere, X, pp, 207-306, Berlin,