The predaceous diving-beetle (Fig. 67) is well named. He is a diver by profession and is a skilled one. The young of this beetle are known as "water-tigers" (Fig. 68), and their habits justify the name. Their food consists of the young of other insects; in fact it is better to keep them by themselves unless we wish to have the aquarium depopulated. When the tiger has reached his full size, his form changes and he rests for a time as a pupa; then comes forth as a hard, shiny beetle like Fig. 67. [Illustration: _Fig. 68. A water-tiger._] The water-scavenger beetle (Fig. 69), so called because of its appetite for decayed matter, is common in many ponds. It has, like the diving beetle, a hard, shiny back, with a straight line down the middle, but the two can be distinguished when seen together. The young of this beetle look and act something like the water-tigers, but have not such great ugly jaws. [Illustration: _Fig. 69. A water-scavenger beetle._] There are three other swimmers even more delightful to watch than those already mentioned. The water-boatmen (Fig. 70), with their sturdy oar-like legs and business-like way of using them, are droll little fellows. They are not so large as the back-swimmers. Fig. 71 shows a back-swimmer just in the act of pulling a stroke. These creatures swim with their boat-shaped backs down and their six legs up. We must be careful how we handle the back-swimmers, for each one of them carries a sharp bill and may give us a thrust with it which would be painful, perhaps poisonous. [Illustration: _Fig. 70. Water-boatman._] The water-scorpion (Fig. 72) is a queer creature living in a neighborly way with the boatmen and back-swimmers, though not so easy to find. Do not throw away any dirty little twig which you find in the net after a dip among water plants near the bottom of a stream or pond. It may begin to squirm and reveal the fact that it is no twig but a slender-legged insect with a spindle-shaped body. We may handle it without danger, as it is harmless. This is a water-scorpion, and his way of catching his prey and getting his air supply will be interesting to watch. He is not shy and will answer questions about himself promptly and cheerfully. Fig. 72 will give an idea of the size and appearance of this insect. [Illustration: _Fig. 71. A back-swimmer._] No water insect except the big scavenger beetle can begin to compare in size with the giant water-bug (Fig. 73). We may think at first that he is a beetle, yet the way he crosses his wings on his back proves him a true bug. In quiet ponds these giants are common enough, but the boy or girl who "bags" a full-grown one at the first dip of the net may be considered lucky.