The last pair of legs are the swimming organs. The tibia and tarsus are fringed with long stiff hair behind, so as to hold the water when the Beetle swims. A peculiar arrangement of the first joint of the tarsus allows the edge to be presented to the water when the limb is carried forward for the return stroke, thus offering the least possible resistance. This Dr. Sharp has compared to the action of a rower in feathering his oar. There is, however, this difference, which it is well to note. The oar is feathered after the stroke; the Beetle feathers its legs before the stroke. It is the first motion when it begins to swim, and the action is not peculiar to the male. We now come to the third region, the abdomen. Like the thorax it is visibly divided into segments, though the division between them is not so great. Much difference of opinion exists as to the number of segments in the abdomen of a typical insect. Some authorities maintain there are eleven, while others put the number as low as five. This, however, is theoretical rather than practical. It is enough for us to know that the number apparently varies greatly, owing to the coalescence of two or more of the segments. [Illustration: FIG. 26.--Upper surface of abdomen of typical Beetle.] The head in Insects, we have seen, carries the eyes, antennae, and feeding organs. The thorax bears the legs and wings. The abdomen bears no appendages, except in some cases, on the last segment; these are called _cerci_. It may be, however, that the stings of bees and the ovipositors of saw-flies and other insects are modified appendages. On examining the abdomen of Dytiscus we shall probably be struck with the difference in appearance between the upper and the under surfaces. The latter is hard, smooth, and shiny; the former, when the wings are removed, is seen to be covered with felt-like hair. Our interest is with the upper surface. Along the abdomen on each side lie spiracles, stigmata, or openings to the breathing tubes. The first and last are larger than the rest, and their general form can be readily made out with an inch magnifier, and with the half-inch we may get some idea of the detail shown in Fig. 27. [Illustration: FIG. 27.--Spiracle of Dytiscus (magnified).] Dytiscus breathes in this way. Floating up to the top of the water, the end of the abdomen projects above the surface. If one watches the Beetle the wing-cases will be seen to rise a little. The air retained by the felted hairs is given off, and a further supply taken in. Then the wing-cases are lowered again; the Beetle gives two or three strokes with its swimming legs, and descends below the surface to ramble round the tank in search of food. [Illustration: FIG. 28.--Tracheal tubes (magnified).]