322 
Birge—The Crustacea of the Plankton. 
Table VII.— Showing the actual number of Diaptomus caught during 
May. 
1895. 
1896. 
May 4- . 
270 
Mav 2. 
730 
May 7. 
410 
May 4. 
660 
May 12. 
710 
Mav 6. 
980 
May 16. 
780 
Mav 8. 
600 
May 18... 
2,200 
May 9..... 
560 
May 20. 
1,650 
Mav 11. 
1,945 
May 22. 
3,820 
May 15. 
6,110 
May 18. 
10,250 
May 21. 
3,690 
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 and 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. 
