FAUNA OP A SMALL BROOK 
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as belonging to the genus Callibaetis but the species was not de- 
termined. On April 24 these hovered above the water in great 
numbers, their long anal cerci acting as rudders to keep them fac- 
ing the breeze in flight. The nymphs of these genera form pre- 
vailing factors in the free-swimming association. Their leaflike 
abdominal gills provide them with oxygen as they swim about 
or rest upon the vegetation. They are plant feeders. 
Odonata — Dragonflies, — Nymphs of the dragonfly, Gomphrus 
sp ? were taken occasionally in dredging the muddy bottom with 
its leafy debris. These are practically conflned to the bottom 
association. They are predaceous, feeding upon other insect 
forms and have been known to attack and devour tadpoles and 
other forms of similar size. 
Hemiptera. — Two families of the Hemiptera were recognized, 
the Hydrobatidae and the Corisidae. Of the former, commonly 
known as the water stridersi, two species, probably Gerris mar- 
ginatus Say, and Gerris reniegis, were taken. These belong 
wholly to the surface fllm association. They move about over 
the water, supported by surface tension, with great freedom and 
agility. They are predaceous and prey chiefly upon other in- 
sects which have fallen into the water. Upon one instance the 
writer observed a strider seize a Simulium fly but a few seconds 
after it had touched the surface and proceed to transfix it with 
its beak to devour the body juices. Another specimen made a 
meal from a drowning May fly adult. 
The Corisidae, or water boatmen, are free-swimming insects 
and form a part of the free-swimming association, although they 
come frequently to the surface and occasionally may be found 
floating. They are predaceous and for the most part feed upon 
other insects. Specimens of Corisa sp? were taken from the 
larger pools. 
Goleopfera — Beetles. — The observations made in this study in- 
volve beetles of four families : Dytiscidae, Hydrophilidae, 
Staphylinidae, and Heteroceridae. 
1. Dytiscidae. The Dytiscids, or predaceous diving beetles, 
were represented by two species, Acilius mediatus Say and Lac- 
cophiliis fasciatus Aube. These beetles move about with great 
rapidity through the water and form an important factor in the 
free-swimming association due to their predaceous food habits. 
They are obliged to rise to the surface occasionally to renew their 
supply of oxygen, taking a bubble of air down with them under 
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