342 
Birge—The Crustacea of the Plankton. 
The following statement shows the general numerical relations 
of the species, observations beginning in July, 1894: 
Season. 
1894. 
1895. 
1896. 
1897. 
Spring... 
Early summer... 
Tiate slimmer. 
? Abundant.. 
? Ephippia .. 
Few. 
Absent. 
Few. 
Abundant.... 
Abundant.... 
Abundant.... 
Abundant.... 
Adult males 
and females 
Few. 
Very few... 
Increasing. 
Abundant.. 
Autumn. 
Absent. 
Very few. 
Winter. 
Absent. 
Very few. 
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 
