452 W. C. ALLEE AND E. R. STEIN, JR. 



DISCUSSION 



Since this is an inquiry into the problem of a possible relation- 

 ship between the sign of the phototactic reaction and the general 

 rate of metabolism of May-fly nymphs, it is pertinent to question 

 the effectiveness of our means of measuring the rate of the meta- 

 bolic processes. The cyanide-resistance method is best checked 

 by comparing results obtained with it with those given by Ta- 

 shiro's method of determining carbon dioxide production. This 

 has been done for the isopod Asellus communis (AUee and 

 Tashiro, '14), There it was found that the two methods gave 

 the same results in direct tests and that two isopods subjected 

 to daily variations of oxygen tejision for ten successive days with 

 daily quantitative estimations of carbon dioxide production by 

 Dr. Tashiro behaved according to expectation based upon 

 previous experience with the cyanide method. 



Regarding the work with Tashiro 's biometer in measuring the 

 carbon dioxide production, I consider this the best, though by 

 no means the fastest, method of making such comparative car- 

 bon dioxide tests as those recorded in this paper. It is less 

 complicated than the electrolytic determination of the hydrogen 

 ion concentration and more accurate than the colorimetric meth- 

 ods. The instrument is somewhat complicated in appearance, 

 but it is in reality as simple in manipulation as a modern micro- 

 scope equipped with oil-immersion lens, mechanical stage, and 

 camera lucida. I have repeatedly demonstrated end points to 

 coworkers at Woods Hole and even to college freshmen. My 

 only change from Tashiro's technique ('17, p. 109) consists in the 

 use of a low-power binocular in reading end points. 



The sources of error in the method as applied to May-fly 

 nymphs are : 



1. May-fly nymphs are water-dwelling animals and were tested 

 in as nearly a dry atmosphere as possible. 



2. Five minutes or more intervened between the time the 

 nymphs were taken from the water and the first indication of the 

 relative rate of carbon dioxide production. During this interval 

 the nymphs must be picked up, partially dried, and placed in glass 

 containers. 



