When resting in the water they float with their head downward and the end of the abdomen slightly projecting from the water; thus a fresh supply of air is easy to secure. In their food habits these beetles are predaceous, and in addition to other insects, will even kill small fish.

The Water-scavenger beetles are not such perfect swimmers as the Predaceous ones. When the latter makes a stroke in swimming it strikes with both hind legs, while the Scavengers strike alternatingly with the hind legs. Their method of securing and carrying air, as with other water-beetles, is remarkable. In addition to the air reservoir under the wings, they have on the under side of the body large hairy areas which communicate with the one under the wings. All the air spaces are thus in direct communication. The respiratory openings in the Predaceous water-beetles open on the upper side of the abdomen, but in these beetles they are on the lower side and surrounded by short hairs which preserve the air film on the lower surface. When the fresh air supply has been exhausted the beetle comes to the surface, tips the body slightly, so as to bring the region on one side of the body just behind the head, to the surface. The long antenna which is folded backward and reaches to the rear part of the head, occupies an air space in its apical half, and in addition is covered by fine hairs, thus being doubly protected from being wetted. At the moment the beetle reaches the surface, by a stroke of the antenna (on the side which is nearest the surface, the body being tilted), the film from the air space in which the antenna rests is carried upward and outward to the surface of the water, thus forming an opening to the exterior. By movements of the wings, aided by bellows-like contractions and expansions of the body, a fresh supply of air is pumped into the air reservoir.

In speaking of peculiar water insects one must not forget to mention the larva of Donacia. The adult female of this interesting leaf-eating beetle often cuts circular holes in the large leaves of water lilies, and then deposits her eggs at the margin of these holes on the under side.

When the larvae hatch they make their way to the roots, upon which they feed. The really remarkable thing about this larva is how it gets its air supply, as it does not have gills, nor is it known to visit the surface for a fresh supply of air, and yet it has a normal air-breathing system. On the dorsal surface, near the tail end of the body, are two slender, curved, spine-like processes. The air tubes of the body arise from the base of these spines, and spiracular-like openings are found at their base.

Two different views have been advocated to explain how it is possible for the larva to secure air. There seems to be no difference of opinion with regard to the source of the air supply, from the air cells in the root of the plant upon which the larva feeds. One view is that these air spaces in the plant are punctured by the spines and thus the air is taken directly into the air tubes. The other view is that the larva bites a hole into the air space and then, by the aid of the spines, holds the openings at the base of the spine against the air space and thus the air is taken up.

The Back-swimmers are curious little fellows which swim upside down in the water, and by means of their sucking mouth parts, prey upon other small animals. The lightness of their bodies and the large amount of air which they carry with them make it necessary when they wish to remain below the surface to hold fast to some object. Thus it takes constant effort in order to remain below the surface. For this reason it is quite natural that they should very often be found floating at the surface where no effort is needed to maintain their position and where an abundant supply of air is accessible.

Another method of securing fresh air, but differing from that of any of the insects previously mentioned, is by means of elongated breathing tubes, thus allowing the insect to remain submerged and yet secure a fresh supply of air from the surface. This method is used by some predaceous water bugs, as shown in the plate, Ranatra fusca. As with all of our Predaceous water bugs, which have the elongated respiratory tube at the end of the body, the Water scorpion has its fore legs adapted for capturing and holding its prey, which consists generally of small fish and insects. The apical part of the fore leg folds back on the basal part which is grooved on its inner face, as a knife blade folds into its handle. As the slender legs of this bug would indicate, it is not an active swimmer, but crawls about slowly.