360 



Aquatic Societies 



5. The slender larvae of a moth fly (Psychoda alter- 

 uata), its body covered with deflexed spines. 



6. The larvae of an unknown caddis-fly whose cases 

 are composed sometimes of stones, sometimes of moss 



fragments. 



Leaf-d rifts— -In the beds of wood- 

 landTxrboks, there are barriers of 

 fallen leaves, piled by the current 

 upon the bare, obtruding roots of 

 trees. These leaf- drifts have a 

 population of their own, the most 

 characteristic member of which 

 about Ithaca is the huge larva 

 shown in figure 215. This is the 

 larva of the giant cranefly, Tipula 

 abdominal is. Associated with this 

 larva in these water-soaked masses 

 of leaves, are the nymphs of such 

 stoneflies as Nemoura and of such 

 mayflies as Baetis and Leptophle- 

 bia, a few beetles and often many 

 scuds (Gammarus). In the mud 

 behind the leaf -drifts, there are 

 often earthworms, washed down 

 from fields above. 



In the clear pools in upland 

 streams that flow through swampy 

 woods, when the bottom is strewn 

 with forest litter intermixed with brownish silt, there 

 dwell a number of forms that certainly belong to the leni- 

 tic rather than to the lotic societies. Such are the caddis- 

 worms of figure 216. With these are associated small 

 mussels of the genus Sphaerium, squat dragonfly 

 nymphs of the genus Cordulegaster, and climbing 

 nymphs of the genus Boyeria, water-skaters on the 



Fig. 215. Two larvae of 

 the giant cranefly, Tip- 

 ula abdominalis, an in- 

 habitant of leaf-drifts in 

 woodland brooks. 

 Natural size 



