Muttkowski—The Fauna of Lake Mendota. 395 
dence of the larvae, or of the exuviae at the time of emergence, 
notwithstanding very careful search. 
B. Order Ephemeroidea. (Table 5, p. 475.) 
All may-flies are water-breathers during their larval exist¬ 
ence. They seem, for the larger part, to be stenoedaphic,^— 
that is, with a small range of substrata. The lake species are 
for the larger part petrophilous, a few species are essentially 
phytophilous. 
Specimens representing various species of the genus Baetis- 
ca, Baetis, Callibaetis, Siphlurus, and Ephemerella are occa¬ 
sional in the lake, but occur so sparingly that they have not 
been considered in the quantitative estimates. 
Caeiiis diminuta Walker. 
This is a small species, very abundant and quite general in 
its distribution. It is phytophilous and quite uniformly dis¬ 
tributed in the littoral area. Like most other may-flies of the 
lake, it emerges in early June. It is common in fish food. 
Ecdyurus maculipennis Walsh. 
Heptagenia canadensis Walker, H. interpunctata Say. 
The three species named form a conspicuous part of the rock 
community of the lake. Their depressed form, as well as their 
strong claws, makes them particularly adapted to turbulent 
waters. H. canadensis in Lake Mendota has been found 
chiefly in depths exceeding two meters, hence beyond the in¬ 
fluence of the greatest wave action. This indicates that the 
condition of the substratum and not the current is the control¬ 
ling habitat factor for this species. All three are true petro- 
phils of the infra-petral type (i. e., found on the underside of 
rocks). 
The food consists of micro-organisms, Hyalella, some Hy- 
drachnids, and occasionally one of their kind. 
Ephemera varia Eaton. 
This species in its general habitus recalls Sialis, with which 
it appears to divide the distribution area. That is. Ephemera 
is a littoral form, and Sialis begins its distribution at the shell 
zone, where the other leaves off. Ephemera varia is a bur- 
