CHANGES OF PHOTOSENSITIVITY WITH AGE 



593 



carefully and estimate as nearly as possible its direction of crawl- 

 ing. Even then there were several individuals which gave no 

 perceptible directive reaction. A few larvae gave positive re- 

 sponses, but the preponderance of movements away from the 

 light in the great majority of cases could leave no doubt that the 

 general reaction was negative in character. 



3 if S L 7 



Age of Larvae in Days 



Fig. 4 Curves showing the degree of photosensitivity from hatching until 

 pupation of individual C. erythrocephala larvae. 



By the second day the larvae had grown sufficiently so there 

 was little difficulty in handling them. All the measurements 

 except those of the first day may, I believe, be regarded as ex- 

 pressing accurately the reactivity of the larvae. There were 

 almost no positive responses obtained on the second day, or later, 

 and these were for the most part due to asymmetry of sensitive- 

 ness, for a positive response in one trail was almost invariably 

 associated with a strongly negative response in the trail made 

 when the same animal was stimulated from the opposite side 

 (Patten, B. M., '14, pp. 230-233 and 259). In comparison with 



