Cyclops. 331 



The foregoing table gives the average numbers of summer. and 

 autumn for the three years, stated in thousands per square me- 

 ter of surface. 



The table shows an autumnal maximum in October, followed 

 toy a steady decline and a slow one as compared with that 

 which follows the spring maximum. The fall increase is due 

 -wholly to C. brevispinosus and the maximum comes when the 

 lake is at or below 15° C. The decline is occasioned partly by the 

 gales of autumn causing the death of adults, and chiefly by the 

 increasing slowness of development of the nauplii as the tem- 

 perature of the water falls. The eggs are still produced and 

 the nauplii hatched, but the young Cyclops are slower in com- 

 ing forward and the deaths exceed the production of young. 

 Food is present in excess of the demands of the Crustacea and 

 so forms no factor in the decline. 



By the middle of December if not earlier the winter condi- 

 tions are fairly established although the number of the species 

 may continue slowly to decline until February. 



A comparison of the charts showing the curve for Cyclops 

 and that for the total Crustacea brings out the fact that Cyclops 

 is the dominant factor in determining the number of Crustacea. 

 All the peculiarities of the general curves are repeated in those 

 for the genus. Cyclops is absolutely the most numerous species 

 except in the summer, when it is sometimes surpassed by Diap- 

 tomus and Chydorus and less often by Daphnia hyalina. Two 

 •causes contribute to this relative disadvantage of Cyclops 

 in summer. First, the species is unfavorably affected by the 

 warmth of the water; second, it is unable to retire into the 

 cooler and deeper water as it might do in lakes which are habit- 

 able below the thermocline. In such lakes it may well 

 be found that Cyclops leads the number of Crustacea through- 

 out the year. A few observations indicate this to be true for 

 Pine lake, but the facts are not well known as yet. 



Zacharias ('96, p. 54) finds only a fall maximum for C. oitho- 

 noides in lake Ploen. There is a trace of a spring maximum but 

 very feebly marked. Apstein ('96, p. 178), finds maxima in the Do- 

 bersdorfer See in May, September, or July and thinks that the 

 maxima may come at any time in summer. He finds on this 



