The thigh (_f_) is the largest joint, for the obvious reason that it contains the muscles that work the tibia and tarsus, which together form a kind of knife-blade, shutting down into a groove in the thigh, which may very well be compared to the handle of a pocket-knife. This description, however, is not to be taken on trust. It is not enough to read about the groove: we must see it for ourselves, raise the ‘knife-blade’ from the groove, and press it down again, and pass the needle along the groove. If we examine the interior of the groove, we shall find that there is a projection along the bottom, so that a cross section would appear like this--W[image]. The inner portion of the tibia is also grooved; so that when once the prey is seized by this powerful limb, it has no chance of escape. The tarsus (_tar_) is not clearly distinguished from the tibia (_tib_) in the engraving, but it may be distinctly seen as a claw-like appendage in a living or dead specimen. The breathing-tube remains to be examined. It may be detached from the body for more convenient manipulation. When this is done, the tube will be seen to consist of two pieces, each grooved on the inner side and set with hairs, which, as they interlock, prevent the entrance of water. A somewhat similar arrangement occurs in the proboscis or tongue of butterflies. There is perhaps a closer parallel in the antennae of the masked crab, which, under certain conditions, form a kind of breathing-tube, each antenna being joined to its fellow by the hairs with which it is set. There are two other Water Bugs which lend themselves to our purpose very well. Each is popularly called Water Boatman, though that name is better confined to Notonecta, because the insects of this genus ‘row themselves about on their backs with their long feathered legs.’ In the United States they are called Water Cicadae, from the shrill noise they make, probably by rubbing the fore-legs together. [Illustration: FIG. 45.--Water Boatman.] In Fig. 45 the Water Boatman (_Notonec´ta glauca_) is represented as seen from above--a position in which we shall rarely discover it, if we keep it in a small aquarium. In Fig. 46 the same insect is shown swimming on its back, or in the position it assumes when taking in a supply of air. The end of the abdomen and the tips of the feet rest against the surface-film; and at the slightest alarm a vigorous stroke of the last pair of legs sends the insect to a place of safety. From the way in which these insects habitually swim, Mouffet came to the conclusion that it was probable men learned the art of swimming on their backs from them. [Illustration: FIG. 46.--Water Boatman swimming.]