LIGHT REACTIONS — METABOLISM — MAY-FLY NYMPHS 455 



The idea that a certain quantitative change must occur before 

 reversal in light reaction takes place is also supported by carbon 

 dioxide determinations made before the nymphs were exposed to 

 light. In some of these, as, for example, the results shown in 

 columns 5 and 6 of chart 1, the nymph with the higher speed of 

 carbon dioxide production became positive upon exposure to 

 light while the other was decidedly negative. It more frequently 

 happened that both nymphs so treated were negative in spite 

 of the fact that one had a higher rate of metabolism than the 

 other. Evidently the biometer is more sensitive to changes in 

 carbon dioxide production than is the ^ight reaction. 



Strychnine, again, did not cause a high degree of reversals, 

 but it did strongly stimulate the nymphs. There is no question 

 but that this stimulation is as strong as that caused by hydro- 

 chloric acid which caused a high percentage of reversals. This 

 difference in result can only be explained by the assumption that 

 while light reversals are accompanied by changes in metabolism 

 that are probably causal, all such changes do not cause reversals 

 in reaction to light. 



The untreated positive nymphs were found to give off more 

 carbon diox de than untreated negative ones. These negative 

 animals often moved back and forth in the dishes, spending the 

 major part of the time at the negative end. Some nymphs do 

 this more than others. This brings them to the positive end more 

 frequently and gives them more opportunities to come to rest 

 there. A reversal apparently on this plan is shown in column 4, 

 chart 1. It appears that this is one mechanism for reversal to 

 -light and here Ihe relation between metabolic rate and the 

 reversal is obvious. The higher the metabolic rate, the greater 

 the tendency to move back and forth; the more random move- 

 ments, the greater the chance of becoming acclimated to the 

 positive end and of reversing the normal reaction. 



All eversals even under the influence of light alone were not of 

 this type, witness column 3, chart 1. This reversal, like most of 

 those experimentally obtained by the use of chemicals, was not 

 preceded by random movements, but took place as though the 

 animal had suddenly discovered the attractiveness of the positive 

 end and must needs go there even though it died in the attempt 



THE JOURNAL OF EXPERIMENTAL ZOOLOOY, VOL. 26, NO. 3 



