EFFECTS OF DROUGHTS AND FLOODS 105 



the vegetation for support. The adult aquatic insects must creep to the 

 surface of the water to renew their air. The forms that have gills are, 

 at least many of them, dependent upon the vegetation for crawling to 

 the surface to molt the old skin. The crustaceans are forms that cling 

 to the vegetation and the snails must come to the surface for air. Doubt- 

 less this formation should be divided into strata, but our data do not 

 justify such division. 



III. Special Stream Problems (103, 92) 



The first special problem is that of the relations of animals to seasonal 

 changes, to changes in volume of water, amount of silt, shifting of bottom 

 materials, and the seasonal aspects of the vegetation. The second prob- 

 lem of streams is the historic or genetic, which includes the phenomena of 

 the origin of the animals of the stream, their mode of entrance, and the 

 effect of rejuvenation, drowning, etc. 



I. SEASONAL CHANGES 



Streams are more strikingly affected by rainfall and drought than are 

 any other of the aquatic habitats. In extremely dry years streams dry 

 up in the rapids where they have perhaps not been dry for a century. 

 Floods change all the landmarks of the stream bottom and often scatter 

 the animals of the stream over the flood-plain. 



a) Floods. — We found at the side of the high bank of the stream 

 where the water is quiet at low water, the Johnny darter (Boleosoma 

 nigrum), the little pickerel (Esox vermiculatus) , the tadpole cat (Schil- 

 beodes gyrinus), the crayfish (Cambarus virilis), and an occasional 

 Hydropsyche. Here were also an occasional sphaerid mollusk and one 

 or two leeches. 



Caught in a mass of driftwood behind the roots of a tree were case- 

 bearing caddis-worms (Phryganeidae), the black-winged damsel-fly 

 nymph (Calopteryx maculata), the larvae of the black fly (Similium sp.), 

 and two species of May-fly nymphs (one Heptageninae). The last two 

 belong to the swift water, the others to the still water or the pools. 

 During floods the still-water fauna and the swift- water fauna become 

 mixed in the still places. 



At the time of our study there was a growth of rank weeds on the 

 flood-plain. While the stream had been swollen for a long period and 

 had stood higher than at the time of observation, little or no invasion 

 of these weeds by aquatic animals had occurred. Animals evidently 

 react negatively to such bottom and vegetation. 



