322 



Birge — The Crustacea of the Plankton. 



Table VII.— Showing the actual number of Diaptomus caught during 



May. 



1895. 



1896. 



It will be seen that these catches divide very sharply into two 

 sets, the division coming between the 16th and 18th of May in 

 1895 aod between the 9th and 11th in 1896. Catches earlier 

 than those given in the table show the same general character 

 as those given, as also do those taken later. There is no earlier 

 catch which is larger than 1000, nor one later in May smaller 

 than 2,000 in 1895 or 3,500 in 1896. 



There is no reason to think that the increase of numbers is 

 due to small, local aggregations of the species. The increase 

 persists without intermission for long periods of time during 

 all conditions of wind and weather. This alone shows that the 

 large numbers must occur over great areas of the lake. On 

 May 15, 1896, observations were made at different points, and 

 the numbers were found practically constant at a distance of 

 2.5 kilometers in various directions from the regular place of 

 collecting. 



It will be seen that the spring increase came just a week 

 earlier in 1896 than in 1895 — on May 11th and May 18th, re- 

 spectively. This acceleration of development, which was shared 

 by all of the Crustacea, was chiefly due to the higher tempera- 

 ture of the water in the latter year. 



In 1895 the ice went out on April 8th, in 1896 on April 2d. 

 In each year cold and rainy weather followed the departure of the 

 ice and at the middle of the month the temperature of the water 

 was almost the same in both years. 



