MuWkowshi—The Fauna of Lake Mendota. 385 
B. INTRINSIC FACTORS 
Respiration .—Aquatic animals can be conveniently divided 
into surface-breathers and water breathers. The latter obtain 
their oxygen directly from the water. Surface-breatherSj how¬ 
ever^ must come to the surface for the replenishment of their 
oxygen supply. Many surface breathers attach themselves to 
the free surface film by means of hydrofuge bristles, plates, 
and rosettes, and bring their stigmatal opening into contact 
with the atmospheric air; others cling to some object in the wa¬ 
ter, usually a plant, while they push their breathing appara¬ 
tus above the water. In the first case a quiet surface film is 
essential, in the second an emergent vegetation,—two condi¬ 
tions which do not exist in the lake for a continuous period. 
The outstanding feature of lake physiology is the constant 
commotion of the surface' Periods of perfect quiet are ex¬ 
ceedingly rare; for even on the calmest days of summer light 
breezes disturb the surface for short periods. The fairly con¬ 
stant wave-action does not permit the growth of an emergent 
vegetation,—the second essential. The two conditions, quiet 
surface film and emergent vegetation, necessary to the surface 
breather, are absent from the lake—hence an absence of sur 
face-breathers from the faunal complex. 
There are a number of insects which would seem to form 
exceptions to the rule, i. e., some aquatic beetles and Hemiptera. 
Certain of the beetles, however, have readapted themselves and 
have become water-breathers (not in a direct sense) again, as 
will be explained elsewhere. Other forms will readily seize 
oxygen bubbles secreted by plants instead of going to the sur¬ 
face,“—an ingenious method of replenishment which is prac¬ 
ticed also by hibernating surface-breathers under the ice. 
In the lake, this method o£ bubble capture applies to most 
beetles and some of the Hemiptera, Some of the latter, how¬ 
ever, regularly swim to the shore during a blow; as do most 
beetles during prolonged blows, altho they are able to stand 
prolonged submersion. 
Locomoiion.—A second characteristic of the lake habitat is 
that the animals are either dingers directly, or indirectly so 
thru artificial aids, such as burrows, tubes, webs, and other fast¬ 
enings. Free swimmers except fish and plankton organisms, 
are rare indeed. Corethra and the Ceratopogoninae are the 
25~-S. A. L. 
