Factors Determining the Annual Distribution. 361 



gust, and the decline continued steadily through September. 

 Similar conditions of temperature to those of 1896 were found 

 in 1894. 



There is no fall reproductive season for Diaptomus, but as 

 the temperature declines the number of egg-bearing females 

 diminishes, and the number of individuals of the species be- 

 comes steadily smaller. The winter level is reached compara- 

 tively early, in late October or the very first of November. 

 After this level is reached, no increase takes place until after 

 May 1st of the following year. The number however, remains 

 singularly constant throughout the winter, and the individual 

 members are well nourished, containing large quantities of fat 

 at all times during the winter. 



Daphnia hyalina has two great periods of reproduction, in the 

 spring and fall. The ovaries begin to develop before the ice 

 has disappeared from the lake in late February and in March, 

 when the temperature of the water is 2.5° C, or above. 

 A very few of the largest individuals produce eggs at this time, 

 but no considerable number of eggs are found until the temper- 

 ature of the lake reaches 4-4.5° C, which has been about 

 the middle of April. In 1895 the first numerous broods of young 

 Daphnias appeared about the middle of May, when the upper 

 water of the lake had reached an average temperature of about 

 15° C, and the reproductive period lasted until about the 

 middle of June. During this time the number of eggs is con- 

 siderable, usually as many as five and occasionally nine, or 

 even more. These eggs are smaller than those produced in the 

 summer, the yolk is peculiar in color, and in general the eggs 

 resemble more nearly those of the ephippia than the eggs pro- 

 duced in midsummer. About three broods are produced during 

 the month by the females. Toward the end of this reproduc- 

 tive period males appear in small numbers. They never exceed 

 4 per cent, of the total number of the females, and I have never 

 found ephippial females at this season though I have searched 

 carefully for them. 



During the first part of June those females die which have 

 lived through the winter, and at the same time there seems to be 

 a break in the reproductive activity of the species. Whether 



