Mutikowski—The Fauna of Lake Mendota. 449 
outlet in swiftness, and Simulium has established itself in such 
spots, an indication that current may be the decisive factor for 
Simulium, while the chemical composition of the water appears 
to be the only factor which prevents its associates of the Yahara 
spillway from establishing themselves also. 
This situation further indicates that the conditions required 
by even a stenophilic species are relative and that the fluctuation 
of conditions resulting in an intergrading of hibitats is paral¬ 
leled by a corresponding intergrading of the respective biota. 
In their general makeup the associations of the lake, rapids, 
and spring outlets are distinctly related. They may be said to 
form the water-breathing communities. In contrast to them 
may be placed the communities of streams, creeks, ponds, 
swamps, and shallows, as typified by air-breathing animals. 
Appendant Lake Shallows .—Certain parts of the lake have 
shallowed out and have been cut off by means of bars so that 
they no longer need be considered a part of the lake proper. 
Yet these shallows are of enormous importance as food sources 
to fish ,especially to the shore fishes, and as breeding places for 
a great number of species, especially among vertebrates and 
surface-breathing insects. 
The shallowing process, frequently a result of sedimentation 
by inflowing streams or of the swamping due to the invasion 
of an emergent vegetation, and the lessened exposure to molar 
agents favor the abundant growth of an emergent and floating 
flora, such as Lemna, Nymphaea, Castalia, Typha, and others. 
In this respect the outlying lake shallows are akin to the 
swamps which line the Yahara Eiver along its course. It is to 
be expected, therefore, that the fauna of the shallows will also 
be similar to that of the swamps, and that is precisely the case. 
The fauna consists chiefly of surface-breathers of various 
kinds, including vertebrates, but it also includes many of the 
water-breathers found in the lake, especially inhabitants of the 
mud bottom. It is marked by the considerable number of Culi- 
eidae, Chironomidae, Syrphidae, Anisoptera, Hemiptera, and 
Dytiscidae, but it has a very scant representation of Epheme- 
ridae, Trichoptera, Zygoptera, and Hyalellae. Stagnation is, 
of course, very great in the swamps, and this means that to some 
extent anaerobic conditions may prevail on the bottom for short 
periods. This may be an important factor in determining the 
faunal complex. 
29--S. A. L. 
