360 Birge — The Crustacea of the Plankton. 



pear in large numbers until after the lake has fallen below 15° 

 C. The production of eggs and nauplii continues throughout 

 the year, but the development goes on with increasing slowness 

 as the temperature of the lake falls. When the temperature of 

 the lake has fallen below 2.0° C, there seems to be little or no 

 development of the nauplii into young Cyclops, but as the 

 water of the lake warms toward the spring, the development- 

 goes on once more. There is, however, no time in the year 

 when female Cyclops may not be found in considerable numbers 

 bearing eggs. 



In summer the number of Copepoda is smaller than that of the 

 nauplii would lead us to expect. It is fair to conclude that 

 at this time the temperature is higher than the optimum for 

 their development into the adult forms. 



Diaptomus does not reproduce during the winter, although a 

 very few females may be found in late February or March bear- 

 ing egg-sacs. No nauplii of this species have ever been seen 

 during the winter, and the total number seen with eggs has 

 not exceeded a dozen during the three winters of my study. Nor 

 does reproduction begin immediately upon the disappearance 

 of the ice. Females bearing eggs are seen from the middle of 

 April on, but the young Diaptomus do not appear in numbers 

 until the water of the lake, to a considerable depth, is near 15° 

 C. Although the numbers of the species vary through the sum- 

 mer, it remains on the whole more constant during the heated 

 term than any of the species, and the late-summer decline in 

 August is apt to be less marked than in other forms. The number 

 of eggs is less in summer than in spring. It may be as great 

 as 30 early in the season but declines to 10-15 later. In 1895, 

 there was a marked rise in the number of Diaptomus during 

 September, which was not seen in 1894 or 1896. Since in all 

 years food is abundant at this season, we must look for the 

 cause of this exceptional increase in 1895 to the persistence of 

 the warm weather during September of that year. A glance at 

 Figs. 1 and 2 will show that in 1895 the surface temperature 

 of the water remained practically constant through the sum- 

 mer and to the end of September above or near 20°, while in 

 1896 the temperature began to decline about the middle of Au- 



