472 Wisconsin Academy of Sciences, Arts, and Letters. 
muddy ooze of the bottom. At night they migrate into the water 
and occupy the various strata of the lake, even coming to the sur¬ 
face. Thus, they show a diurnal periodicity in habitat, occupying 
the mud in the daytime and the water at night. The young larvae, 
however, are entirely aquatic for a time after they hatch out; that 
is, they occupy the lower strata of the lake, the hypolimnion, in the 
daytime instead of the mud, but they come into the upper strata, 
also, at night. When they are from one third to one half grown, 
they show the same behavior as the full grown larvae, that is, they 
occupy the mud in the daytime and migrate into the water at 
night. 
In table 11 the number of these Corethra larvae per square me¬ 
ter of bottom is indicated by months for the period covered by these 
observations. The first part of this table shows the results obtained 
at station II, and the second part gives the averages by months for 
the five stations located in the deep water. The data for station 
II are given separately because the other stations were not visited 
regularly in the winter. Large numbers of the larvae live over 
winter; in fact, they are more numerous from November to April 
than at any other time of the* year. During this period there is 
no loss from pupation, and the losses from other causes are not 
large enough to reduce the number very materially, so that the 
numbers remain uniformly high during this interval of time. 
The results given in table 11 are shown graphically in figure 1. 
The solid line in this figure represents station II and the broken 
line the averages for the five stations. The vertical spaces in the 
diagram indicate the number of individuals per square meter and 
range from zero up to 30,000; the months are shown in the hori¬ 
zontal spaces. 
Beginning with the results for May, 1916, there is a marked de¬ 
cline in the number of Corethra larvae at station II in the month 
of June, with a further decrease in July; the decrease during the 
latter month was not nearly as great as during the former month. 
No observations were made at this station during the month of 
August in that year, so that the gap from July to September is 
represented by a broken line. In order to complete the curve the 
number for August, 1916, has been assumed as about a thousand, 
corresponding to the numbers found at this station in August, 
1917, and 1918. There was a distinct increase in the number of 
Corethra larvae during September, and the rise continued during 
