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 



