366 
IOWA ACADEMY OF SCIENCE 
THE POOL ASSOCIATIONS. 
The pools, with their large expanse of quiet water, of a depth 
not so great as to prevent the thorough influence of the sun’s 
rays, permit the growth of an abundance of algae and other 
aquatic plants on the bottom and of semiaquatic plants at the 
margin. This richness of plant life and the variety of physical 
conditions make possible a division of the fauna into separate 
associations, such as already have been pointed out. 
Surface film association. — Here the physical features of im- 
portance are the atmosphere as a direct supply of oxygen, and 
the surface tension of the water. The most characteristic form 
is the Water strider, which breathes the air directly, is enabled 
to support itself upon the surface fllm, and takes its food by 
preying upon other insects that have fallen into the water. 
Many forms from the free-swimming association depend on 
the surface for breathing, coming up at intervals to renew their 
supply of oxygen. Tipulid larvae which live in the mud on the 
bottoms must take their oxygen from the air, thus establishing 
a connection between these two associations. 
The bottom association. — Upon the bottoms of the pools has 
become concentrated the silt together with vegetable debris, com- 
posed in a large part in this instance of the decaying leaves. 
Living plants are attached here. 
Dredgings from the muddy bottoms of such pools have brought 
to light such forms as flat worms (Planaria), pond snails, the 
larvae of Chironomous and Corethra, dragonfly larvae and water 
scavenger beetles (Hydrophilidae). Cranefly larvae (Tipulidae) 
also are found in numbers on the muddy bottom but their distri- 
bution is limited to the margins where the water is sufficiently 
shallow to allow them to reach the surface with the breathing 
apparatus located on their posterior extremities. Many of the 
free-swimming forms spend a part of their time on the bottom 
or upon the vegetation growing there. Examples; of these forms 
are May fly larvae, mosquito larvae, larvae of Dytiscid beetles 
and adults of the same. 
The free-swimming association. — The free-swimming associa- 
tion consists of these forms which spend much of their time mov- 
ing about throughout the water and which have access to all 
parts of the pool although they may be taken in temporary con- 
nection with those forms of the other associations proper. The 
