280 



CHANCEY JUDAY. 



prevail in the lower strata of the deeper water. Also, the ab- 

 sence of dissolved oxygen in the hypolimnion serves as a protec- 

 tion to the young larvae which occupy this region in the daytime 

 for a certain period after they hatch out. 



NUMBER IN SHALLOWER WATER. 



The larvae of Corcthra pnnctipennis show a decided preference 

 for the deepest portion of Lake Mendota. In the daytime, they 

 are much more abundant in the mud where the water reaches a 

 depth of 20 meters or more than they are in the shallower areas. 

 It was found that the average number of larvae within the area 

 bounded by the 20 meter contour line was more than three times 

 as large as the average for the region lying between the 8 meter 

 and the 20 meter contours, while the number obtained in areas 

 where the water did not exceed five meters in depth was prac- 

 tically negligible. 



Some three hundred samples were taken in series which ex- 

 tended from the shallow water to the deep water ; that is, from 

 a depth of 8 meters or 10 meters down to a depth of 20 meters. 

 The results of four sets of these observations are shown in Table 

 III. It will be noted that there was a marked increase in the 



TABLE III. 



THE NUMBER OF CORETHRA LARVAE PER SQUARE METER OF BOTTOM AT DIFFER- 

 ENT DEPTHS IN FOUR SETS OF OBSERVATIONS WHICH WERE MADE IN 1917. 



number of larvae correlated with the increase in the depth of the 

 water. On May 15, for example, the sample taken at 15 meters 

 yielded about six times as many as the one at 10 meters, while 



