B60 
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- 
