Withstanding the Wash of Moving Waters 259 



2. Special organs for hanging on to water- swept 

 surfaces. Such organs are the huge grappling claws of 

 the nymphs of the larger stoneflies (see fig. in on p. 

 204) and of the riffle beetles: also powerful adhesive 

 suckers, such as those of the larvae of the net-winged 



midges. 



3 . Form of body that 

 diminishes resistance to 

 flow of the water. This 

 we have already seen is 

 stream-line form. In 

 our discussion of swim- 

 ming we pointed out 

 that the form of body 

 that offers least resist- 

 ance to the progress of 

 the body through the 

 water will also offer least 

 resistance to the flow of 

 water past the body. So 

 we find the animals that 

 stand still in running 

 water are of stream-line 

 form ; darters and other 

 fishes of the rapids; 

 mayflies, such as Siph- 

 lurus and Chirotenetes ; 

 even such odd forms as the larvae of Simulium, which 

 hangs by a single sucker suspended head downwards in 

 the stream. Indeed, the case of Simulium is especially 

 significant, for with the reversal of the position of the 

 body the greater widening of the body is shifted from 

 the anterior to the posterior end, and stream-line form 

 is preserved. Such forms as these live in the open, 

 remain for the most part quietly in one position and 

 wait for the current to bring their food to them. 



FIG. 159. The larva of the net- 

 winged midge, Blepharocera, dorsal 

 and ventral views. 



