431 
TABLE 4. MONTHLY AVERAGE OF TEMPERATURE FOR THE 
PERIOD NOV., 1909—APRIL, 1910. 
Month. Temperature near Surface. Temperature near Bottom. 
November...... 13.25° C. 
December ...... Ais 
DEAE TA Go ae eee 3.4° C 
February ....... Baraat eu se as tek pater fess ieeeee 2.05° C 
WEN Soe an See 6.7° C 
Z\ ys] Veneers 15.4° € 
Temperature records are not complete for warmer months, but those 
taken indicate that the temperature of 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 in the plant remains that 
cover the bottom of the pond. Their movements are very slow, and usually 
consist in crawling rather than swimming. On Jan. 13, 1909, 4 inch ice, 5 
inches snow, water temperature 2.2° C., a beetle (Vropisternus 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 reduction in the amount of oxygen rather than to low 
temperature. I have not determined the amount ef oxygen present during 
different seasons of the year. Woweyer, the filamentous algze 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 larvee, and 
Notonecta. Two glass jars which were exactly alike, were filled with 
water to the same level. An equal 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 was allowed to protrude from the water. An 
equal number of organisins was introduced into each jar. The surface of 
