The Annual Distribution of the Crustacea. 311 



find abundant food in the deeper strata, the young are unable 

 to develop, and thus the total number of the limnetic Crustacea 

 slowly declines. The insect enemies of the Crustacea, notably 

 Corethra, are also very numerous at this time, but the number 

 of these which I have found is not great enough to account for 

 the decline in the number of the Crustacea, and the increase of the 

 Crustacea begins in September, before the insect larvae begin 

 to decline. I assign most influence to the first and third of the 

 unfavorable influences which I have named. During this time 

 the periodic species are added but their numbers are usually 

 not great until after the first of September. 



The Crustacea in Fall. 



The number of the Crustacea begins to increase with the 



opening of September (compare Figs. 6-9) and the increase 



continues during that month and into October. This increase 



is due in part to the increase in number of the perennial species. 



Daphnia hyalina and Cyclops brevispinosus multiply and reach a 



maximum in late September or in October. To these species 



are added the periodic forms, which are present in August, but 



ordinarily not in sufficient numbers to balance the decline in 



the other species. During September, however, all increase in 



number together, and bring the total number at the fall maxi- 



t' 

 mum to a point more than half as great as that at the spring 



maximum. In 1894 the maximum, 821,000 per sq. meter was 

 reached in the first part of October; in 1895, the maximum was 

 768,000, in the early part of October; in 1896, there were two 

 maxima, one in early September, numbering 1,441,000, of which 

 more than half was due to Chydorus. The other, the fall maxi- 

 mum proper, was 1,368,000 and came in early October, or leav- 

 ing out Chydorus, 1,123,000 in late October. The figures are 

 the semi-monthly averages. The difference in these dates is 

 apparently dependent upon temperature. If October is warm 

 and pleasant, the development of the Crustacea continues longer, 

 and the maximum is greater than under other climatic condi- 

 tions. In all seasons food is present in superabundance at this 

 time of the year. The algae are at a maximum, and are enor- 

 mously in excess of any demands made upon them by the crusta- 



