38G Wisconsin Academy of Sciences, Arts, and Letters. 
only ones among insect larvae, and Corethra is confined to deep 
water while the others seek shelter in algae (Cladophora, 
Spirogyra, Batrachospermum, etc.). 
Food Supply .—Nutrition is of less infiuence on the lake com¬ 
plex than on the distribution within the lake. Phytophaga 
among animals are restricted entirely to the zone of photosyn¬ 
thesis,—while secondarily sarcophaga are restricted in their 
distribution by the same factor. To escape such limitations, 
some species have become necrophagous and coprophagous, and 
may occur wherever organic waste is present. 
Currents .—It is essential to consider the relative strength of 
currents and their frequency in discussing the lake complex. 
Many water-breathers are restricted to waters which have a 
contin\ious flow, such as rapids and falls; these species are 
homoiophilous. In the lake the habitat which most nearly ap¬ 
proximates this condition is the rock shore, and the gravel 
beach and rachion to a lesser extent, where intermittent takes 
the place of continuous wave action. Hence it is not surpris¬ 
ing that many of the inhabitants of the rock beaches are found 
also in the rapids. 
This statement must not be made too categorically, however, 
for the condition of the bottom may perhaps be a more decisive 
factor than the current. In the case of the inhabitants of the 
rock beaches it may be the presence of rough surfaces, espe¬ 
cially rocks, that is really the determining feature. For in¬ 
stance, Psephenus lecontei, certain may-flies, and other petro- 
philous species are found on the stones of rock beaches and in 
the Yahara Rapids. If current were the decisive factor, then 
they ought also to be found in the short rapids of Merrill 
Springs and Merrill Creek; yet they are absent from these 
places. 
Special Habits .—Certain special habits seem to influence the 
lake complex. This is notably the case among Tri- 
choptera, Ephemeridae, and Odonata. Several species of cad- 
disflies are confined to the springs along the lake shore, never 
occurring in the lake proper. The same is true of some Ephe¬ 
meridae. The dominance of a habit, however, is best illus¬ 
trated in the case of Odonata; here the habit of oviposition is, 
I believe, decisive. Most Zygoptera, Aeschninae, and some 
Corduliinae oviposit in plants,—therefore a habit necessitating 
the presence of emergent vegetation which is of course absent 
