346 
Birge—The Crustacea of the Plankton. 
August, and in the latter part of the month both retrocurva and 
hyalina were practically equal and their numbers rose together 
during September and October. It is quite possible also that 
the lower temperature of the water in September, 1896, as com¬ 
pared with the same month in 1895, favored the development of 
both species. In 1895 the summer temperature of the lake was 
maintained until late in September. The result of this was 
apparently a great increase in the number of Diaptomus , and a 
steady decline in the number of Daphniae. 
D. retrocurva first appears in the latter part of May. The 
numbers are small, but two or three specimens can be found by 
search in almost every catch. During June it apparently disap¬ 
pears, or is much more rare than on this first appearance. It 
is not possible to estimate its numbers with any accuracy be¬ 
fore July or August. The males begin to appear in late Sep¬ 
tember or in October. They were first noticed on September 
17th, 1895, and October 1st, 1896. The ephippia developed 
during October, and the species declines rapidly in November, 
and finally disappears from the lake by January 1st. The ephip. 
pia float, and many of them are doubtless driven to the shore, 
so that if the level of the lake is much lower in the spring and 
summer than it was in the fall, these ephippia may fail to develop, 
and thus cause a scarcity of the species. 
The maximum of this species coincides with the presence of 
the males. These, when at their greatest abundance number 
from 18 to 50 per cent, of the full number caught. They are 
always more abundant, relatively, in the upper strata of the 
water than are the females, agreeing in this particular with 
the young of most species of the limnetic Crustacea. 
The food of this species agrees with that of the other members 
of the same genus. It eats Anabaena and diatoms in prefer¬ 
ence to other plants. It makes very little use of Ceratium and 
avoids Clathrocystis whenever possible. 
Marsh (’97, p. 210) assigns the maximum of D. Kahlbergiensis 
to late October, thus agreeing with the corresponding species 
in lake Mendota. He does not say anything about males and 
since the species was present during the winter of 1894-5 it 
would seem to belong to the perennial Crustacea of Green lake. 
