364 Birge — The Crustacea of the Plankton 



during the latter part of August, rising rapidly to a maximum, 

 in September (Fig. 20), and then declining slowly until late 

 October, when it fell off more rapidly and finally disappeared, 

 with the exception of occasional scattered individuals. In 1895 

 it reappeared in May, reached a small number which it main- 

 tained about six weeks, rose rapidly to a maximum in July, 

 and then declined to a small number which was maintained with 

 approximate constancy from the latter part of August, through 

 the autumn and winter, declining, but not quite disappearing, 

 in the following April. In 1896 the species was much more 

 abundant than in either year, a fact which I have connected 

 with the greater abundance of Aphanizomenon during that 

 season. The species had a great development from July to the- 

 middle of October, reaching its maximum early in September. 

 There was also a minor maximum in early July and one in the 

 first half of October. It appears, therefore, that the maxima of 

 this species have come in July, 1895, in September, 1896, andi 

 in October, 1894, and that in other years these months have' 

 been marked by the presence of very small numbers of the 

 species or its total absence in other years. It is, therefore, 

 impossible to say more on the relation of temperature than that 

 the maxima fall in the warm season of the year. During the 

 winter of 1895-6, when the species was regularly present, re- 

 production went on, as was evidenced by the regular presence 

 of eggs in the brood-sac of the females. 



Summing up these results of temperature, it may be said that 

 in lake Mendota, temperature exerts a greater control over the* 

 number of the plankton Crustacea than does food. The number 

 of the Crustacea falls off in autumn, while food is still abundant; 

 reproduction is checked in winter, although the food present 

 would permit reproduction; and the reproductive periods of the 

 perennial species are arranged rather with reference to temper, 

 ature than to food supply. 



If I were to sum up my impressions as to factors affecting 

 the numbers, I should state them as follows: 



1. Food sets an upper limit to number. 



2. The algae of the upper strata of water determine the de- 

 velopment or failure of the young broods. 



3. Temperature determines the rhythm of reproduction. 



