396 The University Science Bulletin. 



isfactorily establish the relationship which existed between these 

 two forms. 



During the summer of 1921 the writer made another trip to Estes 

 Park, Colo., and again found these May-fly nymphs in the same 

 situation in the Big Thompson river where they had previously oc- 

 curred. A careful search was then made over a distance of a mile or 

 more in this stream, but there was only this one particular spot in 

 which the Rithrogena nymphs could be found in considerable num- 

 bers. This was in a part of the stream where the current was quite 

 swift and the water not over six to ten inches deep. The bed of the 

 stream was covered with stones, many of the stones projecting 

 above the surface of the water. May-fly nymphs, stone-fly nymphs, 

 caddis worms and other swift-water forms were very plentiful. As 

 many as five or six specimens of the Rithrogena nymphs occurred on 

 a single stone. Upon taking a stone out of the water and turning it 

 over, these nymphs would quickly glide to the under side of the 

 stone, appearing to be much more elusive than any of the other spe- 

 cies of the May-fly nymphs present. A total of nearly 300 of the 

 Rithrogena nymphs were collected, and more than ninety per cent 

 of them were found to carry either a larva or a pupa of the chi- 

 ronomid. 



An attempt was then made to rear to the adult stage the May-fly 

 and the chironomid. In order to accomplish this a number of the 

 nymphs which carried pupsB were placed in small wire cylinder 

 cages. Some of the cages were placed in the part of the stream 

 where the nymphs naturally occurred, while others were placed in 

 a spring near the writer's cottage, where they could be kept under 

 close observation. Three males of the chironomids emerged on 

 August 7, two of them from cages in the stream and one from a cage 

 in the spring. A female also emerged on the same date, but it es- 

 caped. On August 8 two males of the May-fly emerged, one from 

 a cage in the stream and the other from a cage in the spring. On 

 the previous day two females of the May-fly were caught in a net 

 near the same spot where all the material was collected, and these 

 later proved to belong to this same species. 



The necessity of leaving the park on August 8 prevented any fur- 

 ther rearing work, and the material from the cages was added to the 

 alcoholic specimens and taken back to the laboratory at Cornell 

 University. 



The May-fly has been determined by Dr. J. G. Needham, who 

 recognizes it as a new species of the genus Rithrogena. A descrip- 



