Muttkowski—The Fauna of Lake Mendota. 393 
able numbers found, the optimum conditions appear to be those 
of the Yahara River, entering the lake on the north. In the 
lateral parts of the stream, where it shallows over the mud of 
the swamps but still retains a fair current, Ceratophyllum and 
Myriophyllum are very abundant and line the channel with ^ 
thick waving mats. These ‘‘mats” form the feeding and 
breeding ground of Hyalella; among the leaves and stems of 
the two plant species they live in such tremendous quantities 
that the plants appear gray from the living encrustation. On 
an area approximating a square meter with a depth of one-half 
meter I collected between seven and ten thousand Hyalellae 
one August day. 
Hyalella is avidly eaten by fish, insect larvae of various 
kinds (Odonata, Gyrinus, Dytiscidae, Hydrophilus), and also 
by leeches. On their own part they eat diatoms, and other 
microfauna and microflora, and will not spurn eggs of Hy~ 
draclmids, damselflies, Trichoptera, and other insects. In 
aquaria I have repeatedly seen them gnawing Chironomid 
pupae, even when other food was plentiful,—on several occa¬ 
sions resulting in the death of some cherished specimen which I 
was attempting to breed. 
Cambarus propinquus Girard. 
This is the commoner of the two species of crayfishes listed. 
Young specimens form an item of the spring food of fishes. 
They are not at all uncommon in the stony and gravelly por¬ 
tions of the shore. There they may be found wedged under 
stones, with smaller pebbles heaped around them,—a type of 
incipient burrow. In the less exposed depths they are not so 
cautious and seek only the slight shelter of plant leaves. C. 
virilis Hagen, another species, seems to be restricted to the 
deeper portions of the lake, where Prof. Pearse has secured 
it in gill nets. 
Asellus communis Say. 
I have only a single record of this isopod in the lake, and 
this, I believe, was an accidental occurrence after a freshet 
which carried much of the Merrill Creek fauna from its orig¬ 
inal location. Its absence from all lake catches is rather pe¬ 
culiar, for it has been reported from European lakes. It is 
very comonm in the slack water of Merrill Creek and the Ya¬ 
hara Spillway. 
