285 



Since the aquatic insects of these collections are being studied by 

 others, with reference to publication in this Bulletin (see Hart, '95, 

 and Needham and Hart, '01), only passing notice of the more 

 important representatives appears in this connection. 



EPHEMERIDA. 



Ephemerid larvae, as a rule in early stages, were found singly 

 or in small numbers in the channel plankton in the warmer months, 

 April-October, at temperatures above 56. Since these occurrences 

 were with few exceptions in stable hydrographic conditions, it seems 

 probable that the younger larvae of this order may adopt, at least 

 temporarily, a limnetic habit. Specific identifications of these 

 larvae were not made. 



HEMIPTERA. 



Corisa (?) sp. Average number, 37. A small hemipterous 

 larva doubtfully referred by Mr. C. A. Hart to Corisa, was taken 

 with some frequency but in relatively small numbers in the plankton 

 during the summer months. Of the 36 occurrences 27 fall in 

 June-August, 2 in May and 3 in September, 2 in January, and 1 each 

 in October and November. It thus appears in the temperature 

 extremes, but exhibits a great predominance in the season of maxi- 

 mum heat. There is no marked increase in its frequency or numbers 

 in years of more disturbed hydrographic conditions. Its numbers 

 are always small and somewhat erratic. Adult Corisa, as well as 

 many other aquatic Hemiptera, were found in plankton collections 

 singly and infrequently. 



DIPTERA. 



This group of insects is abundantly represented in the plankton, 

 but in all cases by larval or pupal stages. 



Chironomus spp., larval stages. Average number, 124. Larvae 

 in various stages of development from that immediately after 

 hatching to that approaching pupation were found in channel 

 plankton. They occur in considerable numbers in the ooze in the 

 river bottom, but appear to abandon the limicolous for the limnetic 

 habit, temporarily at least, as a result of hydrographic or other 

 disturbances. There is evidence from their relative numbers in 



