In Fig. 44 we have a representation of the raptorial (front) leg of this insect. This leg should be compared with the legs of other Insects—not in pictures only, but in actual specimens—that we may see how, while the general plan is preserved, different joints are modified to suit the special function of this limb—that of taking prey. (See also Fig. 43.)

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—. 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.

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.

Active as the Water Boatman is, it often falls a victim to the Water Scorpion, if both are kept in the same aquarium. I learnt this fact by experience, for having put two Water Boatmen into a small tank in which was a Water Scorpion, I found both the former dead in the morning. It was evident that their destroyer had had a good meal.

The only point to which attention need be called in examining the Water Boatman is its method of taking in a supply of air. In looking at a dead specimen we shall see a ridge or keel running down the middle of the under side of the abdomen, and fringed with hair on each side. A similar fringe runs along each side of the abdomen, thus forming two passages along which the air taken in at the end of the body is conveyed to the spiracles, the largest of which are on the thorax.

Corixa (Fig. 47) swims with its back uppermost, and when kept in the aquarium may generally be seen foraging for small creatures—larvae or worms—among the sediment at the bottom. Bateman, who kept a number of these insects, says that he fed his specimens on garden worms and pieces of raw meat. Mine have always foraged for themselves, and done very well without feeding. They may often be seen to rub the short fore-legs alternately across the front of the head, probably for the purpose of producing a call-note. I have often watched them at this practice, but have never been able to detect any sound. The defect is evidently in my sense of hearing, for the sounds rest on undoubted authority, and are coincident with the rubbing of the fore-legs across the head.

Where sound-producing organs exist it is more than probable the capacity for receiving sound-impressions also exists. Graber made some interesting experiments to test the sense of hearing in Corixa. His results are thus summarized by Sir John Lubbock[28]: ‘He placed some Water Boatmen (Corixa) in a deep jar full of water, at the bottom of which was a layer of mud. He dropped a stone on the mud, but the insects, which were reposing quietly on some weeds, took no notice. He then put a piece of glass on the mud, and dropped the stone on to it, thus making a noise, though the disturbance of the water was the same. The Water Boatmen, however, at once took to flight.’