278 CHANCEY JUDAY. 



TABLE II. 



THE NUMBER OF YOUNG CORETHRA LARVAE PER CUBIC METER OF WATER AT 

 DIFFERENT DEPTHS OF LAKE MENDOTA ON AUGUST 2, 1917. 



Those obtained at 10 meters and 12 meters were recorded as very small 

 and for the other depths the individuals were estimated to be from a quarter 

 to a third as large as full grown larvae. 



Depth in Temperature, Number of Larvae 



.Meters. Degrees C. per Cubic Meter. 



8 19-8 O 



10 17-4 44 



I2 l6.0 67 



15 14-5 20 



18 13-6 489 



20 13-5 3ii 



23 13-3 22 



Lake Mendota. In the former lake two net catches on May 25, 

 1917, which were made in the deepest water, namely, 14 meters, 

 gave an average of 422 full-grown C. plumicornis larvae per 

 square meter of surface, while two hauls of mud at the same 

 place yielded an average of 433 individuals per square meter. 

 That is, these larvae were substantially equally divided between 

 the water and the mud at about 10: oo A.M. on a bright morning 

 when the water was so transparent that a white disc 10 centi- 

 meters in diameter did not disappear from view r until it reached 

 a depth of 8.6 meters. In other words, the day distribution of 

 the larvae of C. plumicornis was practically the same in Devils 

 Lake as the nocturnal distribution of the larvae of C. punctipennis 

 in Lake Mendota. 



While the larvae of Corethra punctipcnms give a prompt nega- 

 tive reaction to light, yet it hardly seems probable that their ex- 

 tensive depth migration in Lake Mendota, even including a 

 descent into the mud, is a simple light phenomenon. The trans- 

 parency of the water is usually low in summer; a white disc 10 

 centimeters in diameter generally disappears from view at a depth 

 of two meters to about four meters at this season of the year, 

 which indicates that the light is cut off rather rapidly by the upper 

 strata of water. On the morning of June n, 1920, for example, 

 the disc reading was 4.25 meters. A pyrlimnimeter has been used 

 to determine the rate at which the sun's energy is cut off by the 

 upper strata of the lake. The results obtained with this instru- 



