Factors Determining the Annual Distribution. 359 



half of October. The first ephippial females were seen on Octo- 

 ber 1st, 1895, and October 12th, 1896. By the middle or last 

 of October nearly all the females bear ephippia, and the ephippia 

 are cast off before November 1st. After this date the species 

 rapidly declines, and the last females practically disappear about 

 the first of December, although scattering individuals may re- 

 main until after January 1st. The sexual period of this species, 

 therefore, instead of coming, like that of Diaphanosoma, when 

 the temperature of the lake is still in the neighborhood of 20°, 

 does not begin until the temperature has fallen below 15°. It 

 should be remarked that in all these cases of an autumnal sexual 

 period, scarcity of food can play no part in bringing it on. At 

 this time the lake is crowded with algae of those species which 

 are most greedily eaten by the Crustacea, and in the case of the 

 Daphnias there is always present a large mass of food material 

 between the legs. 



Leptodora is closely parallel to Daphnia retrocurva, although 

 of course, its numbers are far smaller. I have never been able 

 to see the nauplius of this species, though I have looked for it 

 carefully. The young females appear late in May. The species 

 reaches a maximum in late August or September. The males 

 appear in late September or early October, and the species dis- 

 appears about the middle or last of November. 



In the perennial species the effect of temperature is chiefly 

 seen in its action upon reproduction. Cyclops brevispinosus is 

 by far the most indifferent to low temperatures. Its chief re- 

 productive period is in the spring, and the young may appear 

 during the winter beneath the ice, when the temperature of the 

 water is below 3.0° C. The rate of reproduction increases as the 

 lake warms, but the maximum of the species is reached by the 

 time the surface of the water has been warmed to 15°. During 

 the summer the species makes no marked recovery from the 

 spring decline. In Pine lake this species is found during the 

 summer in great numbers, close to the thermocline, living 

 chiefly in the colder water just below it. It seems probable, 

 therefore, that the species is unable to reproduce rapidly in the 

 warm water of lake Mendota, to which it is confined during the 

 summer. The young of the fall reproductive period do not ap- 



