431 



TABLE 4. MONTHLY AVERAGE OF TEMPERATURE FOR THE 

 PERIOD NOV., 1909— APRIL, 1910. 



Month. 



Temperature near Surface. 



Temperature near Bottom. 



November 



Dejember 



January 



13.25° C. 

 4.° C. 

 3.4° C. 

 .3 



2.05° C. 



March 



April 



6.7° C. 

 15.4° C. 





Temperature records are not complete for warmer mouths, but those 

 taken indicate that the temperature oi: the water approximates closely the 

 average diurnal temperature of the air, which data are given in detail on 

 page 425. 



Most of the aquatic beetles of this pond hibernate as imagoes. After 

 the freezing weather comes they are to be found iji the plant remains that 

 cover the bottom of the pond. Their movements are very slow, and u.sually 

 consist in crawling rather than swimming. "On Jan. 13, 1909, i inch ice, 5 

 Inches snow, water temperature 2.2° C, a beetle {Tropistenms mixtus 

 Lee.) was watched for 20 minutes. It was crawling on a Typha stem and 

 during this time left it but once, swimming away a few inches and then 

 returning. 



It may be argued that this quiescent state of the larger forms in the 

 pond is due to the I'edaction in the amount of oxygen rather than to low 

 temperature. I have not determined the amount of oxygen present during 

 different seasons of the year. However, the filamentous alg^e which are 

 present all winter certainly produce some oxygen and it is highly probable 

 that the Typha stems allow some gaseous interchange to take place be- 

 tween the air above the ice and the water below it. I have made the fol- 

 lowing simple experiment with beetles (5 species), Corethra larviie, and 

 Notonecta. Two glass jars which were exactly alike, were filled with 

 water to the same level. An etpial amount of Typha stems was placed in 

 each. In one, the stems were completely submerged, while in the other 

 one, the end of each stem \\as allowed to protrude from the water. An 

 equal number of organisms was introduced into each jar. The surface of 



