Diaptomus. 
321 
the reproduction of the Crustacea in winter is slower than that 
of the algae. It is not impossible that the slight decline in 
numbers noticeable in 1895-6 may be attributable to the multi¬ 
plication of Cyclops in that winter. The decline in Diaptomus 
is too small to allow of certainty in the inference, but the adult 
Cyclops fell off rapidly in March of that year as they did not in 
the preceding winter when little reproduction took place. Food 
also became much more scanty in the spring of 1896 than in the 
preceding year. The amount of food material in the spring of 
1895 was estimated as at least four times as great relatively to 
the number of Crustacea present. 
The chief enemies of the Crustacea are the larvae of insects and 
the young fish, both of which are absent or few during the winter. 
Leptodora also, though living chiefly on Cyclops and Daphnia , 
must devour some Diaptomi during the summer; while it is 
wholly absent in winter. At this season the perch, which also 
feed on the small Crustacea, are at the bottom and apparently 
do not feed at all. There seem therefore to be no enemies of 
the Crustacea during the winter and their numbers are corres¬ 
pondingly constant. 
Throughout this season also Diaptomus is fat — fatter than in 
summer, as the drain on tissue for reproduction is absent. 
In April after the ice breaks up the Crustacea are wont to de¬ 
cline in numbers. This is especially true for those species whose 
reproductive period comes somewhat late in the spring, and in 
which only the individuals which have lived all winter are pres¬ 
ent in the spring. These find the conditions of the open water 
of the early spring harder than those under the ice, especially 
as they are exposed to the competition of the increasing swarms 
of Cyclops and sometimes of D. pulicaria. The smallest catches 
of Diaptomus which are met during the year, are obtained in 
the latter part of April when the number of Cyclops has risen 
greatly—more rapidly than the food has increased. 
In May there comes a great increase in the number of Diapto¬ 
mus. It shows itself first by the presence of a great number of 
immature animals in the upper strata of the water. In both 
years the appearance of these new members of the species was 
very sudden, as will be seen from the following table. 
21 
