Insects and their world 



effect is to throw the beetle into the air, when it has a chance of alighting 

 on its feet. 



Swimming 

 Some aquatic insects move only on the surface of the water, some move 

 with their bodies almost, but not quite submerged, and the majority 

 hve below the surface, entirely surrounded by water. 



The first group are the 'skaters', and are supported by the surface- 

 tension of the water (Fig. 19). To break through the surface of water 

 requires a definite force, the amount of which is in proportion to the 

 area to be broken. If an object is placed on the surface its weight tends 

 to drag it down, and the surface tension to support it. Weight depends 

 on volume, surface tension on area. Since a small objea has a surface 

 area large in proportion to its weight, some inseas are able to walk on 

 the water without breaking through. Those which practise this often 

 increase the effect of surface tension still more by covering the body 

 with fine hairs, or branching filaments, and by making the body surface 

 oily or waxy so that it is difficult to wet. 



The skaters are carnivorous bugs of the Hemiptera-Heteroptera. The 

 water-measurers (Hydrometridae) are stick-like insects with long legs, 

 and crawl about on the water just as if they were on dry land. By 

 contrast, the true pond-skaters (Gerridae) use the two middle legs 

 together, in a leaping, or rowing movement, steering with the trailing 

 hind-legs. 



The whirligig beetles (Fig. 5 id), which live on the surface with only 

 their backs exposed, propel themselves by using the submerged legs 

 like oars. The middle- and hind-legs are broader than usual, and grip 

 the water well; the fore-legs are used for seizing the prey on which this 

 carnivorous insect feeds. 



The underwater swimmers— water-boatmen (Fig. 5ie), back- 

 swimmers and water-beetles— also use the hind-legs as oars, moving 

 them together in the second, or jumping movement, not alternately as 

 in running on land. These Hmbs, and sometimes the middle pair as 

 well, are broader than usual and have fringes of long hairs. In action, 

 they are turned edge-wise on the forward stroke, just as an oar is 

 feathered. 



All these insects take a supply of air down with them as a bubble. The 

 water-scorpions (Nepidae) on the other hand, stay within reach of the 

 surface with their long siphon, and simply walk about on the submerged 

 plants, as if they were on land: they have no special modifications, 

 except for hydrostatic organs which warn them before they go too deep 

 for their siphon to operate. 



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