TABLE 5- 

 as detern 



Site and distribu 

 ined by class studii 



vertebrate populations 



Number per square meter 



...Iter, they show a larger thorax and legs, a smaller 

 alxlomen, absence of hair on the caudal cerci, shorter 

 middle cercus, and smaller gill lamellae. These modi- 

 fications enhance body streamlining and reduce the 

 drag of the water. Furthermore, the legs are articu- 

 lated in a way which allows the current to press them 

 hrmly against the substratum. The body itself swings 

 freely in the current. 



Mayfly naiads that occur in quiet waters do not 

 lia\e these modifications. They commonly spend 



most of their time in burrows, dug into the mud. 

 They come out at night to swim around and search 

 for food. The abdomen of the mud-inhabiting forms 

 is thick, with little taper, sometimes bowed vcntrally, 

 and the three terminal cerci are provided with long 

 stiff hairs that overlap and make an excellent oar for 

 swimming. 



Stonefly naiads are not limited to stony habitats. 

 Some species occur in the masses of leaves that lodge 

 against rocks or along the banks, in the algae grow- 



Streams 45 



