427 



(I am informed that Dytiscus marginaJls Linn was formerly obtained from 

 ttiis pond in quantity for laboratory dissection material.) I have never 

 talien a specimen from the pond. What other forms may have been elimi- 

 nated by this "drying up," I do not know, because I began to study it at 

 this period. 



Not only were the conditions during this period of low level very dif- 

 ferent from those preceding it. but llie conditions after the dry period were 

 also very different. 



When the pond began to fill with water in November, 'OS, the decaying 

 amphibian larvre and other organic matter developed conditions favorable 

 to the production of an enormous number of flagellates. This decaying or- 

 ganic debris and possibly the flagellates farnished an immense amount of 

 food for some of the Crustacea, especially Cijpricloi)sis vidua Brady. The 

 algie are eaten by both the amphibian larvie and Simoceplialus vetelliis. 

 The elimination of the former greatly increased the food material of the 

 latter. 



The dragon fly nymphs and possibly Corethra larvae feed on on both 

 of these crustaceans. Thus the conditions at this period furnished the 

 Crustacea an enormous food supply and few enemies. The result was a very 

 great development of Crustacea. Especially was this true of CryprkJopsis 

 viihm Brady. Since the winter of 190S, conditions which I have not been 

 able to determine have resulted in the entire elimination of this form. It 

 is evident that variations in the level may result in the elimination of a 

 si>ecies or its abnormal development. 



Temperature. — The seasonal development of different forms as indi- 

 cated in the list and table, is probably due directly or indirectly to changes 

 in temperature. The temperature in t,he water of the pond varies from 

 27.8'' C to at the surface (Ice) and to l..S° C at the bottom. 



Except for the first few vreeks the temperatures were taken with a 

 centigrade thermometer graded to l/.'jths. The winter of 1908-1909 was 

 fairly open. Ice formed December 2, lasting until January 20. Ice was 

 present the latter part of February but there was none after March 3. 

 The maximum thickness of ice for this year was 2.5 inches. The winter 

 of 1909-1910 was very se^'ere for this latitude. Ice formed December 7 and 

 lasted until March 2 and had a maxnnuni thickness of 9 inches on January 

 11. During the first wniter, the temperature of the water a few Inches 

 under the ice, varied but slightly from the greatest density temperature. 



