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. 



