The Annual Distribution of the Crustacea , 811 
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¬ 
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- 
