Juday—Diurnal Movement of Plankton Crustacea. 553 
was two meters deeper in October than, in August in Oconomo- 
woc lake and it was found that the day position of D. pulicaria 
was two meters deeper. The same results were obtained in 
Okauchee lake with the exception that both were three meters 
deeper in October than in August. About the middle of June, 
it was found much nearer the surface because only a compara¬ 
tively thin stratum of water had become too warm for it. 
Also Birge (2b, p. 401) found that the center of population for 
this form in Lake Mendota rose from fourteen meters in late 
September to four meters in the first part, of November, wdiile 
in the latter part of December 1 it lay only about two meters be¬ 
low the surface. He also states that a reversal of this diagram 
“would serve fairly well to indicate the downward migration 
of the species in the spring.” There was no diurnal movement 
of D. pulicaria in Lake Mendota, but the above serves to show 
how temperature would have affected the extent of such a daily 
migration if there had been one. 
In coming to the surface in Oconomowoc lake in August, this 
form moved up into the water that was at least 15° 0. warmer 
than that occupied by it during the daytime. In October, the 
difference in temperature did not exceed 7° or 8°. In Okau¬ 
chee lake, the difference in temperature was not So great in 
August as in Oconomowoc lake but it was about the same in 
Oetober. 
In Oconomowoc lake, adult D. pulicaria appeared at the sur¬ 
face during the second half hour after sunset and then disap¬ 
peared during the second half hour before sunrise. In Okau¬ 
chee lake, they were not found at the surface in August and 
October, but in' June they came to the surface about two hours 
after sunset and left again during the fifth half hour before 
sunrise. In both lakes, the young preceded the adults in the 
evening by a half hour to two hours. In one instance, the young 
left the surface about the same time in the morning as the adults 
and again at least an hour later than the adults, 
Daphnia longirenns Sars. 
This species was found in only one lake, Okauchee. There 
was no diurnal movement. 
