OBSERVATIONS ON LARWE OF CORETHRA PUNCTIPENNIS. 275 



the area of the lake lies within the 18 meter contour. For two 

 or three weeks in August, in fact, a little more than half of the 

 lake bottom is subject to anaerobic conditions. No attempt was 

 made to ascertain the effect of this lack of oxygen on the devel- 

 opment of the Corethra eggs. Many cocoons of the Oligochaet 

 Limnodrilus w r ere noted in the bottom material during this period, 

 however, and the eggs in them seemed to be developing normally 

 in the absence of oxygen. This fact suggests that the eggs of 

 Corethra may also develop normally under anaerobic conditions. 

 The young Corethra larvae were not noted in the series of net 

 catches until the last week in June, though the eggs of the first 

 adults each year undoubtedly hatch at an earlier date than this. 

 They were found regularly in the net catches from the latter part 

 of June to the first week in October. 



BEHAVIOR. 



In 1917 and 1918 net hauls were made regularly at three of 

 the stations before the mud catch was taken in order to see if 

 any of the full-grown larvae occupied the water in the daytime, 

 but the results were entirely negative. Some of these hauls were 

 made as early as 8 : 30 A.M. and others as late as 4:30 P.M., so 

 that these observations covered the chief portion of the day. It 

 was found also that the full-grown larvae deserted the water on 

 cloudy days as well as on clear days. 



A series of observations made at Station II. during the after- 

 noon and evening of July 16, 1917, showed that the full-grown 

 Corethra larvae had not emerged from the mud by 7 : 30 P.M., or 

 just about sunset. At 8:00 P.M., or half an hour after sunset, 

 133 larvae and 88 pupae per square meter of lake surface were 

 found in the water. By 8:30 P.M., or one hour after sunset, 

 these numbers had increased to 3,945 larvae and 442 pupae. At 

 the latter hour the full-grown larvse had reached the surface of 

 the lake, thus showing a vertical migration of 23.5 meters during 

 an interval of about one hour. 



A similar set of observations was made in 1920 beginning at 

 5 :45 P.M. on June 10 and continuing until 5 :3O A.M. on June 

 ii. On the former date sunset came at 7:36 P.M., standard 

 time, and sunrise on the following day at 4:18 A.M. No Co- 



