Cyclops. 
381 
The foregoing table giyes 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 
<by 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 
