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 3 1 collections 

 examined, averaging 88 per m. 3 , while in 1898, in more disturbed 

 conditions, there were 29 occurrences in 52 collections, averaging 

 124 per m. 3 There is also a marked seasonal distribution. The 

 larvae 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 is 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 larvae have the characteristics of limnetic 

 organisms, and may be reckoned among the autolimnetic 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. 3 to 4 

 in 1898, in pump collections, averaging 8 per m. 3 Corethra larvas 

 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. Larvas were 

 recorded singly in scattered occurrences in all months but February 

 and October-December, though most of them appear during maxi- 

 mum temperatures. 



Larvae of Tanypus and Odontomyia 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- 



