HOW INSECTS SWIM 117 



"The body resembles a boat. There is nowhere a projecting point or a 

 sharp corner which would offer unnecessary resistance to motion ; bulging out 

 in the middle and pointed at the ends, it cuts through the resistance of the water 

 like a wedge. The movable parts, the oars, seem to be as well fitted for their 

 purpose as the burden to be moved by them. That the hind legs must bear the 

 brunt of this follows from their position exactly in the middle of the body, where 

 it is widest. In other insects also these legs are used for the same purpose as 

 soon as the insects are put in the water. But the swimming legs of water- 

 beetles are oars of quite peculiar construction. They are not turned about in 

 the coxce, as an- vtlii'r leys, but at the foot-joint. The coxa, namely, has grown 

 entirely together with the thoracic partition. The muscles we have mentioned, 

 exceeding in strength all the soft parts taken together, take hold directly of 

 the large wing-shaped tendons of the upper thigh, and extend and retract the 

 leg in one of the planes lying close to the abdominal partition. The foot 

 forms the oar, however. It is very much lengthened and still more widened, 

 and can be turned and bent in by separate muscles in such a way that in the 

 passive movement, that is, the retraction, the narrow edge is turned to the fore, 

 and therefore to the medium to be dislodged ; however, as soon as the active 

 push is to be performed and the leg is extended with greater force, it cuts down 

 through the water with its whole width. These effective oar-blades are still 

 considerably enlarged by the hairs arising on the side of the foot, which spread 

 out at the decisive moment. 



"Every one knows that the oar-blades of swimming beetles always go up 

 and down simultaneously and in regular time. On the other hand, as soon as 

 one puts a Dyticus on the dry land, i.e. on an unyielding medium, it uses its 

 hind legs entirely after the manner of other land insects ; that is, they are drawn 

 in and extended again alternately, as takes place clearly enough from the foot- 

 steps in Fig. 119, A. We learn from this that water insects have not yet, from 

 want of practice, forgotten the mode of walking of land insects. 



" The forcing up of the water as a propelling power is added to the repulsion 

 produced by the strong strokes of the oars. If the beetle stood up horizontally 

 in the water, he would be lifted up. 



" As the trunk, however, assumes an oblique position when the insect wishes 

 to swim, one can then imagine the driving up of the water as being divided 

 into two forces, one of which drives the body forward in a horizontal direction, 

 while the other, that is, the vertical component, is supplied by the moving of the 

 oars. The swimming insect is thus, as it were, a snake flying in the water. 



"The long streamer-like hind legs of many water-bugs, for example Noto- 

 necta, approach more nearly our artificial oars. These legs are turned out from 

 the bottom. 



" There is no doubt but that the legs of insects, as regards the many-sidedness 

 and exactitude of their locomotive actions, place the similar contrivances of 

 other animals far in the shade. We shall be forced to admire these ingenious 

 levers still more, however, when we take into consideration their energy and 

 strength. That the force with which the locomotive muscles of insects is drawn 

 together is enormous compared with that of vertebrates, we may learn if we try 

 to subdue the rhythmical movements of the thorax of a large butterfly by the 

 pressure of our finger or to open against the insect's will the closed jumping leg 

 of a grasshopper, or the fossorial shovel of a mole-cricket." 



