320 
Birge—The Crustacea of the Plankton. 
Table VI .— Diaptomus. Average number expressed in thousands per 
square meter of surface. 
1894-5. 
1895-6. 
1896. 
67.6 
148.4 
52.8 
October 16-31. 
38.3 
79.7 
48.8 
November 1-15. 
44.0 
55.8 
29.8 
N ovember 16-30.. 
46.0 
28.5 
December 1-15. 
23.9 
33.6 
29.3 
December 16-31. 
(16.7) 
58.0 
24.7 
Jannarv 1-15. 
17.5 
48.6 
.Tanuarv 16-31. 
(15.9) 
28.3 
Fehrnarv 1-15. 
(44.5) 
38.9 
February 16-31. 
28.0 
35.0 
March 1 -15. 
28.3 
March 16-31. 
34.7 
33.3 
April 1-15. 
14.0 
35.2 
April 16-30. 
20 6 
29.9 
May 1-15. 
34.4 
102.3 
May 16-31. 
207.9 
360.2 
Numbers enclosed in a parenthesis rest on observations made on a single day during 
the half-month. 
These figures show that Diaptomus begins to decline toward 
its winter condition early in the autumn. There is no marked 
reproductive period in the fall which supplies the individuals 
which are to live over winter, but the numbers steadily and 
rather rapidly decline after the time when the lake has decid¬ 
edly cooled from its summer temperature. The table also shows 
that the mortality must be very small in winter. In spite of 
the fact that there is no reproduction, the numbers show very 
little decline after the winter conditions are fairly established, 
and only a slow decrease in the late autumn. Indeed from the 
middle of October until the first or middle of May, the semi¬ 
monthly averages show no more variation than might easily 
appear in two catches made on the same day at the same place. 
This persistence of the numbers of the species must be attributed 
to the absence of competition and of enemies during this season. 
The food supply is ample for the winter stock of Crustacea and 
