Juday—Quantitative Studies of Fauna in Lake Mendota. 477 
of 390 larvae per square meter. These averages indicate that about 
400 larvae per square meter survive the winter. The number re¬ 
mains above 300 individuals per square meter until June; follow¬ 
ing this is a marked decrease in July, with a further decline in 
numbers in August. The minimum number was found in Septem¬ 
ber. Following this, there was an increase, with the number ris¬ 
ing above 300 per square meter in December. The majority of the 
samples obtained in September contained no Protenthes larvae, and 
similar results were obtained for many of the samples procured in 
late August and early October. 
Table 14 gives data concerning the weight of these larvae, to¬ 
gether with the percentages of water and of ash. The results for 
the large specimens (9 millimeters to 10 millimeters long) repre¬ 
sent the averages of four sets of weighings. The rest of the table 
shows the results obtained when all of the specimens found in a 
sample were weighed. These individuals ranged from 5 millime¬ 
ters to 10 millimeters in length, the minimum length increasing 
somewhat between autumn and the following summer. The growth 
of the smaller individuals is well illustrated m the four weighings 
that were made between December 21, 1917, and June 10, 1918; 
the average weight of the mixed sizes almost doubled during that 
period of time. The maximum weight of the mixed sizes (June 
10, 1918) was less than half as much as the average weight of the 
large size. The percentages of water and of ash were also smaller 
in the mixed sizes than in the large specimens. 
The mean of the weights obtained for the mixed sizes in Febru¬ 
ary and May gives a fair average for the larvae which survive the 
winter; that is, live weight 1.4 milligrams per larva and dry weight 
0.238 milligram. On the basis of these weights, the April crop of 
larvae, namely, 400 individuals per square meter, represents a live 
weight of 5.6 kilograms per hectare (5 pounds per acre) and a dry 
weight of approximately one kilogram per hectare (0.8 pound per 
acre). Whether there is more than one generation per year has 
not been determined, so that no estimate of the annual production 
can be given for this form. The winter larvae, however, repre¬ 
sent a comparatively small amount of material, so that an addi¬ 
tional generation or two would not increase the food supply in 
this region very materially. The above figures, also, do not in¬ 
clude the various losses sustained during the winter, but these 
losses doubtless represent only a small percentage of the annual 
crop of larvae. 
