As would be expected, some of the most interesting things about these animals are connected with modifications of their form which have resulted from their aquatic life. It is believed that the ancestors of water insects have been land insects which invaded the water and have thus become greatly modified in their new surroundings. Locomotion and breathing, either one or both of these functions, are, as a rule, very different in land and water insects. The variety of aquatic insects, if we consider only the adults, is not great when compared with the land insects. But when we compare fresh and salt water forms it is surprising how few kinds there are which live in the sea, in spite of its vast area and great food supply. So few are the insects found in the sea, or other salt waters, that, to most of us, to speak of aquatic insects only calls to mind fresh water forms. We shall, therefore, refer almost wholly to fresh water forms. Let us consider briefly a few examples of these. We may distinguish two general groups, according to their special habitat. Belonging to the first group are those insects which frequent, primarily, the surface of the water. These forms which breathe air directly, and not air dissolved in water, as is the case with many other water insects, must be kept dry and be able to maintain their position on the surface of the water. Surface insects, such as the Water-Skaters, found on quiet ponds and streams, and their marine relatives, _Holobates_, accomplish this by means of fine hairs which cover the feet where they touch the water. The same physical principal is involved here, as when a needle or wire is floated upon water,--that of surface tension. The fine hairs on the body of a water insect act in the same way as those on the feet, and thus keep the insect dry when below the surface. These insects are thus able to breathe as land insects, on account of their being on the surface, and consequently their respiratory systems are not as greatly modified as in many of the insects living beneath the surface. It must be borne in mind that an insect breathes by means of the air which enters the body by small openings and is led by means of tubes, which become very finely divided, like veins, to all parts of the body. By means of contractions and expansions of the body of the insect, the air within these tubes is caused to circulate, and thus impure air is driven out and a fresh supply is pumped in. Two of the commonest of these surface dwellers, so well known to the small boy who frequents ponds and streams, are the Whirligig-beetles or Lucky-bugs, and the long-legged Water-striders or Water-skaters.