LIGHT REACTIONS — METABOLISM — MAY-FLY NYMPHS 453 



Regarding these points it must be borne in mind that the 

 readings here given are all comparative ones in which the control 

 and experimental nymphs were treated exactly alike. Since one 

 must needs be taken from the water before the other, this was 

 varied in the different experiments. The nymphs often lived 

 twenty-four hours in the apparatus and at times they lived 

 as long as forty-eight hours in the slight amount of moisture 

 present. 



3. Difference in carbon dioxide production may be due to 

 difference in movement. This source of error was eliminated 

 by the simple method of throwing out all tests where movement 

 occurred. 



4. Unconscious personal preference for one of the nymphs 

 producing more carbon dioxide. This was eliminated by the 

 experimenter's ignorance of which was experiment and which 

 was control. 



The relationship between the general metabolic processes 

 of animals and their reaction to light may conceivably be one of 

 the following : 



1. Conditions that depress positive animals may make them 

 negative (Mast, Bohn, Drzewina) and conditions that stimulate 

 negative animals may make them positive (Holmes, Carpenter, 

 Bohn, Jackson). 



2. The above relationship may be reversed (Phipps in part). 



3. Conditions that stimulate animals may cause reversals of 

 their normal reaction or vice versa. 



4. Conditions that markedly change the metabolic rate may 

 cause reversal of either positive or negative animals. 



5. Changes of metabolism accompanying changes in light reac- 

 tions may be incidental or resultant rather than causal. 



6. The relation between metabolic processes and the reaction 

 to light may vary in different species so that no general law can 

 be worked out. 



7. There may be no relationship between the rate of metabol- 

 ism and the phototactic reaction. 



"With the positive Epeorus nymphs only depressing agents 

 were used and these caused reversals. With both the negative 



