288 A NATURALIST IN THE GREAT LAKES REGION 



resistance to the water, and many of them have specially devel- 

 oped suckers, by means of which they attach to the rock. The 

 water penny (Fig. 445) is so altered by its adaptation to its 

 environment that one would never take it at first sight for a 

 beetle larva. 



The snail, Goniobasis livescens, lives in the rocky rapids, 

 dozens of them often on a single bowlder, clinging on by their 

 strong muscular foot, feeding on the microscopic plants that 



FIGS. 443-447: Fig. 443. Net-building caddis-fly larva, Hydropsyche, in its 

 tube (above); front view of tube (below), enlarged; Fig. 444. Coiled tube of 

 larva of caddis-fly, Helicopsyche, enlarged; Fig. 445. Water penny, larva of 446; 

 Fig. 446. Brook beetle, Psephenus lecontei; Fig. 447. A common rotifer, 

 much enlarged. 



form the slimy layer of green scum on the rocks. The liberty 

 cap snail, Ancylus, whose coiled shell has been reduced to a 

 broad, low cone, is fairly abundant in the stony rapids of the 

 brooks. 



Pleurocera elevatum is found where the current is not so very 

 strong. It often prefers the gravelly margins of the stream. 

 The crayfish, Cambarus virilis, is found hiding under the stones, 

 very commonly. 



