Factors Determining the Annual Distribution. 359 
half of October. The first ephippial females were seen on Octo¬ 
ber 1st, 1895, and October 13th, 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- 
