Muttkowski—The Fauna of Lake Mendota. 445 
found in mineral waste eontaining a concentration of 5.5% 
NaCl. 
Chironomids have been found so regularly in drainage and 
sewage water (they are constantly mentioned in reports on 
sewage) that it has been suggested that they play an important 
role in sewage disposal,—indeed, a part akin to the sewage bac¬ 
teria. Such a role appears to be held by the Chironomids and 
worms of the aphytal area of the lake. They are scavengers 
in the broadest sense, even coprophagous, and that they may aid 
if not entirely take the part of, the sewage bacteria in the lake 
seems not unlikely. 
Part IV. COMPAEISONS AND SUMMARIES. 
In discussing lake life, the prime question confronting us is: 
^‘What are the advantages of aquatic life, particularly of lake 
life?” Why have so many insects reverted to acquatic life? 
It is easy to understand why animals which are holaquatic in 
the first place should remain so, but once a terrestrial mode of 
life is acquired a change is more difficult to understand. 
Except for Odonata, Ephemeroidea, Plecoptera, and some 
Neuropteroidea, aquatic life is secondary among insects. That 
is, while the insect progenitor was aquatic, and the oldest in¬ 
sects were amphibious, their descendants largely adopted a ter¬ 
restrial mode of life. The above mentioned orders, however, 
appear to have retained their primitive amphibious life. Other 
orders, such as Diptera, Coleoptera, Hemiptera, and Lepidop- 
tera, became holoterrestrial, but many of their modern repre¬ 
sentatives have again adopted an amphibious life. 
As to the why and wherefore, we have not far to seek. A 
terrestrial mode of life entails adaptations to a series of ex¬ 
tremes. Sudden and often extreme changes of temperature, of 
winds, variations from drouth to fiood, from dust to mud, and 
the like, are all of a kind to require considerable adjustment on 
the part of the organism. When we consider the additional 
need of food and of shelter from enemies, we cannot wonder 
that so many insects,—indeed, the greater number of them— 
have acquired some means of procuring additional shelter at 
least for the larval stages. 
