CHANGES OF PHOTOSENSITIVITY WITH AGE 



589 



that the records of the same individual could be compared from 

 day to day; and (2) a group of 35 larvae kept in a common cul- 

 ture and tested each day in the manner described, but without 

 keeping separate the records of individuals. All the larvae were 

 kept in the dark throughout the experiments. By daily obser- 

 vations on these two sets of larvae it was possible to ascertain 

 both the changes taking place in the photosensitivity of individual 

 animals, and by averaging the reactions of all fifty maggots the 

 changes taking place in the photosensitivity of the culture as a 

 whole. 



RESULTS 



The results of these experiments might be expressed either 

 graphically or by means of tables. The tabular method has the 

 advantage of being more complete, but where the results are as 

 simple as they are here, it is hardly necessary to give a complete 

 tabular statement of all the measurements. Therefore only a 

 single condensed table is given showing the average responses of 

 the fifteen individuals kept isolated throughout the series of 

 experiments (table 2) . There were a few casualties among the 

 larvae due probably to excessive handling in the very young 

 stages. In all cases the dead larvae were replaced by fresh in- 

 dividuals of the same age from a gross culture kept for the pur- 

 pose under precisely the same temperature and moisture condi- 

 tions as the experimental cultures. Replacements are marked 

 on the table by an asterisk. 



A very clear idea of the changes in sharpness of reaction which 

 take place with age, is given by the series of actual trails photo- 

 graphed in figure 2. The trails were selected to show as nearly 

 as possible the characteristic reaction for the age they represent. 



