320 



Birge — The Crustacea of the Plankton. 



Table VI.— Diaptomus. Average number expressed in thousands per 



square meter of surface. 



October 1-15 — 

 October 16-31. .. 

 November 1-15 . . 

 November 16-30. 

 December 1-15.. 

 December 16-31. 



-January 1-15 



January 16-31. .. 

 February 1-15. .. 

 February 16-31 . . 



March 1-15 



March 16-31 



April 1-15 



April 16-30 



.May 1-15 



May 16-31 



1894-5. 



67.6 

 38.3 



44.0 



23.9 



(16.7) 



17.5 



(15.9) 



(44.5) 



23.0 



28.3 



34.7 



14.0 



20 6 



34.4 



207.9 



1895-6. 



148.4 

 79.7 

 55.8 

 46.0 

 33.6 

 58.0 

 48.6 

 28.3 

 38.9 

 35.0 



33.3 



35 2 



29.9 



102.3 



360.2 



1896. 



52.8 

 48.8 

 29.8 

 28.5 

 29.3 

 24.7 



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 



