274 CHANCEY JUDAY. 



DEPOSITION AND DEVELOPMENT OF EGGS. 



According to Muttkowski the adults emerge at night, "begin- 

 ning early in the evening and continuing through the night. In 

 the morning, if the lake is quiet, the females can be seen on the 

 surface, ovipositing through the surface film." Eggs deposited 

 by females kept in insect cages sink to the bottom of the aquarium 

 and experimental evidence indicates that those deposited at the 

 surface of the lake also sink to the bottom. Mud from Station 

 II. where the water is 23.5 meters deep and also from Station I., 

 located in water 18.5 meters deep, was washed through a sieve 

 with meshes fine enough to remove all of the larvae ; this sifted 

 mud w T as then placed in aquaria. At the end of five days a dozen 

 small Corcthra larvae had appeared in the material from Station 

 II., while five small larvae were noted in the other bottom ma- 

 terial at the end of a week, thus showing that the mud from both 

 stations contained eggs. 



Eggs that were deposited by females kept in captivity hatched 

 within forty-eight hours when the temperature of the laboratory 

 ranged from 21 to 24 C. The temperature of the lower water 

 in the deeper portions of the lake is much lower than this, how- 

 ever, and the eggs which reach the bottom in these areas probably 

 do not develop so rapidly. The bottom temperature at Station 

 II. in summer, for example, ranges from slightly more than 9 

 in some years to about 14 in other years. On the other hand, 

 eggs deposited in water not exceeding five meters in depth are 

 subject to temperatures of 20 to 25 in July and August so that 

 they probably hatch about as promptly as those kept under lab- 

 oratory conditions. 



Another factor that may retard development in the deeper 

 water is the absence of free oxygen. Usually all of the dissolved 

 oxygen below a depth of 18 meters is used up by the middle of 

 July, after which no oxygen is available in this region until Oc- 

 tober. Thus, all of the Corethra eggs which reach the bottom in 

 water that is 18 meters deep or more during this period must de- 

 velop under anaerobic conditions if they develop at all. This 

 anaerobic stage covers the greater part of the most active repro- 

 ductive period of this insect and approximately 30 per cent, of 



