EFFECTS OF DROUGHTS AND FLOODS IO5 
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. Spectat 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 (Szmilium 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. 
