276 CHANCEY JUDAY. 



rethra larvae were obtained from the water on June 10 until 7:15 

 P.M., at which time a catch yielded 22 individuals per square 

 meter of lake surface. Fifteen minutes later, or just a few min- 

 utes before sunset, this number had risen to 176 larvae per square 

 meter, and by 7:50 P.M., or about a quarter of an hour after 

 sunset, the number was 1,576. At 7:36 P.M. the larvae had not 

 invaded the upper 10 meters of water, but they had ascended to 

 the 10-15 nieter stratum. They did not appear at the surface 

 until about an hour and a quarter after sunset, so that the rate of 

 upward migration was somewhat slower than that noted in 1917. 

 Pupae reached the surface at 9:00 P.M., or approximately an 

 hour and a half after sunset. The largest number of both larvae 

 and pupae found in the water during this set of observations was 

 noted in a catch taken at 10:00 P.M.; of the former there were 

 4,730 individuals per square meter of lake surface and of the 

 latter 287. By 11:00 P.M. the numbers had declined to 2,100 

 and no respectively; the numbers were substantially the same as 

 these at 2:00 A.M. on June 11. 



Larvae were still found in the upper meter of water at 3 : 30 

 A.M., but they had disappeared from the upper 10 meters by 

 3:47 A.M. and only one individual was obtained in a catch taken 

 from the 0-15 meter stratum at 3:50 A.M. Practically, then, 

 they deserted the upper 15 meters of water during a period of 

 about 20 minutes. It should be noted, also, that this downward 

 migration was not due to direct sunlight since it took place at 

 least half an hour before sunrise. The larvae were still occupy- 

 ing the 15-23 meter stratum in considerable numbers, since a 

 catch at 3:55 A.M. yielded 1,658 individuals per square meter 

 in that region ; the same catch contained 88 pupae per square 

 meter also. The number of larvae in the lower water then gradu- 

 ally diminished, the last disappearing between 4 : 45 and 5 : 

 A.M. According to these results, then, the full-grown Corcthra 

 larvae enter the bottom mud by the end of the first half hour after 

 sunrise and they remain there until about sunset. 



Samples of mud taken at 6:00 and at 7:00 P.M. on June 10, 

 1920, yielded an average of 2,720 larvae and 55 pupae per square 

 meter of bottom ; as a result of the migration into the water these 

 numbers had declined to 1,400 larvae and 22 pupae per square 



