246 INSECT WATER TRESPASSERS. 



very perceptible, and the difference between the two 

 forms is just that of the keeled rowing-boat and the 

 keel-less canoe. 



Another very remarkable point is the structure of 

 the rowing legs, or oars, as they may be called. The 

 last pair of legs are reserved for this purpose, the first 

 pair being chiefly used for catching prey, the middle 

 pair for crawling, and the last for swimming. I 

 strongly recommend the reader to capture one of these 

 insects, which may be found by hundreds in every 

 pond, and to examine the structure of their swimming 

 legs. I cannot but think that they gave the first idea 

 of oars, as used at the present day, i.e., propelling 

 instruments set at right angles to the central line of the 

 boat, having broad ends and a very powerful leverage. 

 The superior power of the oar over the paddle is the 

 reason why the latter has been gradually abandoned 

 in favour of the former. 



Now, in the leg of the Water Boatman all the ele- 

 ments of the oar are found. 



Firstly, there is the leverage. An oar is a lever, 

 the fulcrum of which is the rowlock, the shorter arm 

 being within the boat, and the longer outside it. The 

 shorter arm being pulled by the rower's hands, causes 

 the longer to describe a considerable arc in the water, 

 and the end of the oar being flat and thin, so as to 

 offer the greatest possible resistance with the least 

 weight, the boat is urged onwards. 



Now, let us take the Water Boatman, and see how 

 the rowing legs of this insect comply in every respect 

 with the boat-oar which has been slowly developed 

 through a series of years. In the case of the insect, 



