282 CHANCEY JUDAY. 



between the shallow half and the deep half of the lake are (i) an 

 active migration, (2) the currents direct currents on the wind- 

 ward side of the lake and return currents on the leeward side, 

 (3) a relatively greater loss in the shallow water due to pre- 

 datory enemies. 



NUMBER IN OTHER LAKES. 



For purposes of comparison similar quantitative studies of the 

 bottom population were made regularly in Lake Monona and in 

 Lake Waubesa during 1917. The former is only one kilometer 

 from Lake Mendota and has nearly as great a maximum depth, 

 namely, 22.5 meters. Lake Waubesa lies about seven kilometers 

 southeast of Lake Mendota, but it is a much shallower body of 

 water, having a maximum depth of only a little more than u 

 meters. In Lake Monona only about one tenth as many Corethra 

 larvae were found as at corresponding depths and times in Lake 

 Mendota ; in some instances the difference was more than a hun- 

 dredfold in favor of the latter lake. In the deepest part of Lake 

 Waubesa the number varied from about the same as that at 1 1 

 meters in Lake Mendota to only a third or a quarter as many ; but 

 the deeper water of Lake Mendota yielded from forty to a hun- 

 dred times as many larve as the deepest portion of Lake Waubesa. 

 Bottom material has been obtained from about a dozen other Wis- 

 consin lakes and in all of them the Corethra population has been 

 relatively small, which seems to indicate that Lake Mendota offers 

 a particularly favorable habitat for these larva?. 



GRAVIMETRIC RESULTS. 



Between June, 1916, and April, 1917, more than fourteen 

 thousand larvae of Corethra punctipennis were picked out of the 

 material collected in Lake Mendota and they were dried for the 

 purpose of making a chemical analysis of them. The average 

 amount of dry matter per individual for this number was 0.251 

 milligram. The average weight was also determined for eleven 

 other lots of larvae containing from 100 to 300 individuals each. 

 These averages ranged from a maximum of 0.311 milligram per 

 larva, dry matter, in June to a minimum of 0.182 milligram in 

 early September. The results of these weighings are shown in 



