CHANGES OF PHOTOSENSITIVITY WITH AGE 595 



about, as if seeking a favorable spot for pupation. If the muscu- 

 lar system is more effective at one time than at another, we should 

 certainly expect it to be during this period of its greatest activity. 

 It is therefore most probable that the decHne in reactivity is not 

 due to decrease in muscular efficiency. It is equally improbable 

 that the coordination between receptor and effector is lowered 

 during a period of increased effector activity. Though this does 

 not constitute proof that the drop in reactivity is due to a de- 

 crease of photosensitivity in the receptors, it points strongly 

 toward that interpretation. 



The decline in photosensitivity has been said to occur in the 

 migratory period. ' We may be more specific. Figure 3 shows 

 that reactivity begins to drop between the fourth and the fifth 

 day and continues to drop steadily until the seventh day, there- 

 after remaining about constant till pupation. On the fourth day 

 all the larvae were feeding. Between the fourth and the fifth 

 day 15 per cent of the larvae had migrated, by the sixth day 60 

 per cent had migrated and by the seventh day practically all the 

 larvae had left the food. The coincidence of the drop in the re- 

 action curve with the onset of migration in the cultures is most 

 striking ; whether or not there is any correlation between the two 

 I cannot say. One is tempted to suggest that there may be, upon 

 the cessation of feeding, a reduction in the rate of metabolism of 

 some photolysable substance. Such a suggestion is, however, 

 of little value except as a hypothesis for further work. 



It would be interesting, also, in view of the change in the sign 

 of phototaxis taking place in the blowfly during the pupal stage, 

 to ascertain if the drop in the curve of negative photosensitivity 

 exhibited during the last stage of larval life, were carried on by 

 an increase in positive photosensitivity during the early life of 

 the imago. 



Whatever may be the underlying cause, there can be no doubt 

 that larvae of C. erythrocephala react to light very differently at 

 different ages. Observations which I carried out on a few larvae 

 of Lucilia sericata show^ed that they exhibited changes in photo- 

 sensitivity similar to those demonstrated for C. erythrocephala. 

 Although the number of individuals handled was not sufficiently 



