358 Birge — The Crustacea of the Plankton. 



or five times as much as in mid-summer. During this period 

 the conditions of temperature are by no means unfavorable for 

 reproduction, and it is at present impossible to see why Crus- 

 tacea should not increase more rapidly and thus reach a greater 

 number at the period of the fall maximum. 



Temperature. 



The temperature of the water, as such, independent of its in- 

 fluence on the food supply, determines the reproductive powers 

 of the Crustacea and the rate of their development, and thus 

 limits their numbers. Perhaps, also, it exerts an influence on 

 the length of life of the adults, although this influence is less 

 certain. 



The different species of limnetic Crustacea differ greatly in 

 their relation to temperature. The periodic species are neces- 

 sarily more greatly influenced by it than are the perennial. 

 Diaphanosoma brachyurum is the most stenothermous of the 

 periodic species. The first scattering individuals appear late 

 in May but the species does not become a regular constituent 

 of the plankton until late in July or early in August. The species 

 increases in number throughout August and early September. 

 The males appear towards the middle or last of September, when 

 the species rapidly declines and wholly disappears from the 

 plankton before the 1st of November. Its active period, there- 

 fore, lies during the time when the temperature of the water 

 of the lake to a considerable depth equals or exceeds 20° C. 

 The individuals found in October are the survivors of the Sep- 

 tember swarm, which show no reproduction and which disappear 

 rapidly. 



Daphnia retrocurva comes next in its relations to tempera- 

 ture. The species first appears late in May, but develops very 

 slowly, and does not become plentiful enough to be counted as 

 a regular constituent of the plankton until late in July or early 

 in August. Its appearance thus coincides approximately with 

 that of Diaphanosoma, but its autumnal history is quite dif- 

 ferent. The species continues to increase sexually until mid- 

 October. The immature males appear late in September or early 

 in October. The females begin to develop ephippia in the first 



