INSECTS IN THE WATER 311 



liind pairs of legs are paddle-like. They spend most of 

 their time skimming over the surface-film, but dive 

 rapidly when danger threatens. Their larvae are con- 

 tinually submerged, but when full grown they ereep up 

 the stem of a water plant, upon which they spin their 

 small, oval cocoons. 



The best-known water-beetles, however, are the Dutis- 

 cidce, typified by the common diving beetle (Dytiscus 

 marginaUs). This insect's body is oval, somewhat flat- 

 tened, with a highly polished surface, while the various 

 parts are so closely adjusted as to present practically a con- 

 tinuous outline. The great hind-legs are modified to form 

 a pair of oars, which — by a peculiar keel-like prolongation 

 of the thorax — are set far back, below the centre of the 

 insect. This arrangement, and their articulation, enable 

 these limbs to be brought at right angles to the body, 

 thus providing for a strong, wide sweep. Moreover, the 

 joints of the tarsi are flattened and clothed with stiff 

 hairs, so that these organs are transformed into perfect 

 reciprocating oar-blades. Normally, they strike the water 

 simultaneously; but they can be used singly for the 

 purpose of turning in a confined space, just as an expert 

 sculler will use one oar to manoeuvre his skiff. Thus, 

 while the broad structural features of Dytiscus agree with 

 those of its near relations, the carnivorous ground-beetles 

 {Carabidce), they are beautifully adapted to meet the 

 requirements of an aquatic life. Indeed, no other insects 

 are equipped for so wide a sphere of existence as these 

 large water-beetles. They are certainly clumsy pedes- 

 trians, but their broad wings enable them to make long 

 voyages in the air ; and if one pool or stream is not to 

 their liking, they quit it at night, and range over the 

 countryside in search of more congenial quarters. The 

 rapacious larvae are often called " water tigers." They 



