286 
years of different hydrographic conditions that these have consider- 
able influence in bringing them into the plankton. Thus in 1897, 
in stable conditions, there were only 5 occurrences in 31 collections 
examined, averaging 88 per m.°, while in 1898, in more disturbed 
conditions, there were 29 occurrences in 52 collections, averaging 
124 per m.? There is also a marked seasonal distribution. The 
larvee appear in the plankton in March—December through the 
seasonal extremes of temperature, but the numbers in March and 
November—December are always small. Only 15 per cent. of the 
occurrences and 5 per cent. of the individuals were found at tem- 
peratures below 45°. The percentage of occurrences in the collec- 
tions 1s highest in March—September, the percentages being 53, 73, 
80, 47, 78,52, and 50, respectively, to:.8 to 35 per cent. during the 
remaining months. 
Corethra sp., larval stages.—Average number, 6. These semi- 
transparent and active larve have the characteristics of limnetic 
organisms, and may be reckoned among the autolmnetic planktonts 
of our waters. Because of their activity. it seems probable that 
they escape the drawn net,—especially the small model used by 
us,—and also, because of their negative rheotaxis, elude the suction 
of the plankton pump to an even greater extent. Thus, in 1895, in 
net collections, there were 8 occurrences averaging 32 per m.? to 4 
in 1898, in pump collections, averaging 8 per m.* Corethra larve 
were never abundant in our plankton, probably in part for the 
reasons just cited. With two exceptions all the occurrences lie in 
the period of maximum temperatures in June-September, 7 of the 
14 occurrences and one third of the individuals being recorded in 
August. 
Dixa sp., larval stages.—Average number, 8. Larve were 
recorded singly in scattered occurrences in all months but February 
and October-December, though most of them appear during maxi- 
mum temperatures. 
Larvee of Tanypus and Odontomya were also recorded in May 
and June in isolated occurrences. 

In addition to the larval stages of these aquatic insects there 
occurred in the plankton a considerable number of insect eggs, 
principally those of Diptera and Ephemerida. These were generally 
isolated, though sometimes fragments of the egg-string of Chirono- 
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