284 CHANCEY JUDAY. 



cent, of the dry weight to a maximum of 9.5 per cent., the former 

 being noted in February and the latter in September. The pupae 

 and adults yielded substantially the same percentages of ash, but 

 these percentages were much smaller than in the larvae. 



There is more or less overlapping of the summer broods, which 

 makes it difficult to estimate the number of larvae produced during 

 this season, but the winter crop of larvae may be estimated with 

 some degree of accuracy. In this investigation twenty-two 

 samples of mud were obtained at the five regular stations in deep 

 water during the month of November and they yielded an average 

 of 17,350 Corcthra larvae per square meter of bottom. Five 

 samples were secured in December also, and they gave an average 

 of 21,900 individuals per square meter; but four of these samples 

 were taken at Station II. which usually gave a larger yield than 

 the other four stations. According to these figures, the early 

 winter population of Corcthra larvae within the 20 meter contour 

 may be conservatively estimated at 18,000 individuals per square 

 meter. Between October and May the live weight averaged 3.1 

 milligrams per larva and the dry weight 0.266 milligram. Apply- 

 ing these weights to the above population gives a live weight of 

 55.8 grams per square meter, or 558 kilograms per hectare, which 

 is equivalent to 497 pounds per acre, and a dry weight of 4.8 

 grams per square meter, or 48 kilograms per hectare, equivalent 

 to 42.7 pounds per acre. 



Muttkowski states that there may be two generations of 

 summer larvse in addition to the winter generation ; but, since the 

 former average somewhat smaller in size than the latter, the total 

 weight of the summer broods is probably not greatly in excess of 

 that of the winter brood. That is, a live weight of 1,200 kilo- 

 grams per hectare (1,070 pounds per acre) would be a conserva- 

 tive estimate for the total annual production of Coretlira larvae 

 in the deeper part of Lake Mendota ; on this basis the dry weight 

 would amount to somewhat more than 100 kilograms per hectare, 

 or approximately 90 pounds per acre. These figures apply only 

 to that portion of the lake which lies within the 20 meter contour 

 line, since the larvae are found in very much smaller numbers 

 in the shallower water. 



