LIGHT REACTIOXS METABOLISM — MAY-FLY NYMPHS 427 



REVERSALS OF PHOTOTAXIS 



In experiments upon phototaxis the nymphs were selected from 

 a number which had stood a short time in tap water exposed to 

 the Hght conditions under which the test was to be made. Then, 

 if the experiments were to be upon negative animals, only decid- 

 edly negative nymphs were selected. During the course of the 

 experiments nymphs were subjected to various strengths of sul- 

 phuric, hydrochloric, and acetic acids; potassium and sodium 

 hydroxide; potassium, sodium, calcium and magnesium chlorides; 

 ethyl alcohol, chloretone, caffein, strychnine, and to temperature 

 changes. 



The two species with which we worked reacted oppositely 

 to light. Epeorus was normally positive while Leptophlebia was 

 normally negative. The former is the more definite in its reac- 

 tion, as shown hy the ratios of the average negative (N) and 

 positive (P) responses of the controls. Thus Epeorus with 



P 90 



seventeen sets of control readings gave a ^ ratio of -.-, while 



5 

 Leptophlebia with thirty such controls gave j^- 



Quantitative reversals were obtained with Epeorus with 

 ethyl alcohol, calcium chloride, and with a decrease of tem- 

 perature. Alcohol was the most efficient reversing agent used 

 with this species. At tap temperature (about 12°C.) the best 

 results were obtained with a 2 per cent solution. When the tem- 

 perature was lowered 5° or more, better results were obtained 

 with a 1 per cent solution. Under both conditions quantitative 

 reversals were obtained with positive nymphs and with the 

 control strongly positive throughout. 



With Leptophlebia more of the chemicals tried gave quantita- 

 tive reversals. Hydrochloric and sulphuric acids, potassium and 

 sodium hydroxides, potassium cyanide, and potassium chloride 

 gave 100 per cent reversals of negative nymphs while the controls 

 were predominantly negative throughout. Sodium and calcium 

 chlorides gave 80 per cent reversals with the controls over 80 per 

 cent negative. Magnesium chloride, chloretone, and caffein 

 had little effect. Alcohol, as with the other species, gave a high 

 percentage of reversals, although quantitative results were rare. 



