QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 225 



have not seen this particular feat repeated. Though any 'ex- 

 treme individuahty' of this sort was uncommon, it was apparent 

 that the rate of crawHiig and the response to hght stimulation 

 varied so greatly with different larvae under the same conditions 

 that it had to be taken into account in a comparative study of a 

 large number of individuals. To compare a larva of unknown 

 sensitiveness, tested under one set of conditions, with an equally 

 unknown larva tested under other conditions, would be of little 

 value as an accurate basis for comparing the effect of the con- 

 ditions. If one were attempting to find the precise effect of 

 humidity on the time made by long distance runners, one would 

 not try various runners of unknown speed, each under a different 

 degree of humidity, and compare their 'times' as an index of the 

 humidity effect. The only logical method would be to time the 

 same runner under various conditions. His actual time would 

 be an individual characteristic, but the relative time of the 

 records under different conditions would be an index of the effect 

 of humidity. 



3. Standard test 



But we cannot use this method on blowfly larvae, for the short 

 sensitive period coupled with the rapid changes in the degree of 

 sensitiveness with age, makes it impossible to complete a series 

 of comparative experiments on a single larva. The nearest ap- 

 proach that can be made to such a method is to use, instead of a 

 single larva, individuals which are as near alike as possible. For 

 this purpose a standardization test, as it might be called, was 

 devised and throughout the whole series of experiments only larvae 

 testing to the uniform standard of sensitiveness were used. 



During the experiments, each larva was kept in a separate, 

 numbered box. The record of the standard test, as well as the 

 subsequent trails, was made by putting a drop of very dilute 

 methylen blue solution on the posterior end of the larva and letting 

 it mark its own course on a sheet of paper placed on the obser- 

 vation stage. This method of recording the trails of larvae was 

 used by Pouchet ('72) and Gross ('13). It has been repeatedly 

 checked by control experiments with tap-water and appears to 



