342 



Birge — The Crustacea of the Plankton. 



The following statement shows the general numerical relations 

 of the species, observations beginning in July, 1894: 



Season. 



Spring 



Early summer 



Late summer 



Autumn 



Winter 



1894. 



? Abundant 

 ? Ephippia 



Few 



Absent 



Absent 



1895. 



Absent 

 Few... 



Abundant . 

 Abundant. 

 Abundant . 



1896. 



Abundant 



Adult males 

 and females 



Few 



Very few. 

 Very few. 



1897. 



Very few... 



Increasing. 

 Abundant . . 



As was stated in my former paper, (Birge, Olson, and Har- 

 der, '95, p. 473), this species is found through the summer in 

 the deeper water only. Scattering individuals may be found 

 extending to the surface, but even where one-sixth of the total 

 number of Crustacea was counted, the number of this species 

 found rarely exceeded one individual; and in my studies during 

 1896, no individuals of the species were found from the upper 

 levels of the lake. As will be stated more at length on the sec- 

 tion on vertical distribution, D. pulicaria is confined in lake 

 Mendota during the summer to the space immediately about the 

 thermocline. It is unable to rise higher on account of 

 the high temperature of the water, and is unable to descend 

 lower on account of the impurity of the deeper water in late 

 summer and early autumn. This fact limits greatly the num. 

 ber of the species during the warm season of the year, and in 

 lakes whose bottom water is cold and not contaminated by de- 

 composition products the number of the species is far greater 

 during the summer months, and the period of active sexual 

 reproduction is a much longer one. 



This species varies much more in numbers from day to day 

 than does any other of the species whose numbers are at all 

 considerable. The station at which most of the observations 

 were made was not far from the southern shore. As a result of 

 the action of the wind the thermocline is subject to 

 considerable variation. A violent southwest wind, especially 

 has the effect of driving out the warm water near the bottom 

 of the lake, and thus temporarily raising the temperature of the 



