Fresh-Water Insects 289 



a considerable number pass the whole or the greater 

 part of their lives in the waters of rivers, lakes, and 

 ponds. Among insects which are aquatic in all stages 

 of their existence we can distinguish between those 

 which glide or skate over the surface of the water, 

 diving not at all or only exceptionally, and those 

 which habitually dive and swim through the water 

 after the manner of fishes. The most typical of the 

 surface-dwellers are the bugs (Hemiptera) known 

 as " pond-skaters," and their aUies — the family 

 Hydrometrids. They are long, narrow insects ; the 

 legs of the front pair, comparatively short, serve 

 to seize prey or anchor to a support ; while the 

 greatly lengthened middle and hind-legs, by strong 

 rapid strokes, carry the insect quickly over the 

 surface of the water. If pond-skaters be watched 

 resting on a shallow brook in the sunlight it will be 

 seen that a bright circle surrounds the shadow cast 

 by each foot. These circles indicate refraction of the 

 light-rays due to the depression of the surface film at 

 the points where the feet rest on it ; the film sinks in 

 under the feet but is not broken, its resistance being 

 enough to support the insect's weight. No bright 

 margin surrounds the shadow cast by the body which 

 is thereby shown not to rest on the surface. Pond- 

 skaters are entirely covered with a thick, velvet-like 

 pile ; the hairs are so close together that the surface- 

 film cannot penetrate between them. Consequently 

 when the insect dives an air-bubble forms around it, 

 providing a store of free oxygen for breathing, and 

 preventing any access of water to the spiracles (170). 

 Among the Coleoptera the Whirhgig Beetles 

 (Gyrinidge) frequent the surface of ponds and brooks 

 where they may be seen in small companies, perform- 

 ing a whirling, mazy dance over the surface-film. 

 These insects, when they dive, carry down with 



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