Daphnia pulicaria. 
341 
a new period of development in July, 1897. From this state¬ 
ment of fact it appears that Daphnia pulicaria , as found in 
lake Mendota, has a biennial period of development extending 
from July of one year to July of the next, followed by a year in 
which the species is either absent or its numbers are exceed¬ 
ingly small. The study made in July, 1894, seems to have 
taken the species at the very end of its developmental period 
after the production of the sexual eggs. One entire cycle was 
included from July, 1895, to July, 1896, and a second period 
when the numbers were extremely small, although never en¬ 
tirely absent, from July, 1896, to July, 1897. 
Table XVIII.— D. pulicaria. Average number per sq. meter, expressed 
in thousands. 
1894. 
1895. 
1896. 
a 
88.2 
January 16-31... 
a 
24.8 
February 1-14...... 
a 
64.1 
February 15-28. 
a 
43.9 
March 1-15. 
a 
March 16-31. 
a 
20.9 
April 1-15. 
a 
28.0 
April 16-30. 
a 
118.2 
May 1-15. 
a 
284.9 
May 16-31. 
a 
533.6 
June 1-15. 
a 
168.6 
June 15-30. 
a 
78.2 
July 1-15. 
6.4 
s 
39.3 
July 16-31.. 
8.3 
11.6 
11.8 
August 1-15..... 
1.1 
19.9 
3.7 
August 16-31. 
0.8 
38.1 
5.9 
September 1-15. 
a 
33.8 
23.5 
September 16-30. 
a 
98.2 
3.4 
October 1-15. 
a 
26.9 
0.4 
Octobor 16-31. 
a 
23.5 
a 
November 1-15... 
a 
49.6 
s 
November 16-30. 
a 
58.3 
s 
December 1-15. 
a 
141.1 
s 
December 16-31. 
a 
99.8 
s 
a, absent; s, scattering individuals only. 
