586 BRADLEY M, PATTEN 



When the blow-fly larva first emerges from the egg, it is either indiffer- 

 ent, [or] only slightly negative. . . . As it grows, it becomes more 

 and more responsive to directive light and by the time the feeding period 

 is ended, it is very strongly negative in its response to light. 



Other than these general statements I was unable to find any 

 data on changes in the blowfly larva's photosensitivity. There 

 has, I believe, been no attempt made to deterniine accurately 

 for the larvae of any species of blowfly, the relative sensitiveness 

 at various ages. It was to obtain information on this point that 

 the experiments described in this paper were devised. The prob- 

 lem may be formulated as follows : 



STATEMENT OF PROBLEM 



What, if any, changes take place in the sign or degree of the 

 blowfly's larva's reaction to light during its life from hatching 

 until pupation? 



METHOD 



Blowfly larvae during the age when they are most frequently 

 used for experimental purposes respond to a single horizontal 

 beam of light by crawling away from the source in the direction 

 of the rays. Changes in the direction from which the light acts, 

 induce corresponding changes in the larva's direction of locomo- 

 tion. There is, however, considerable variation in the directness 

 and accuracy with which individual larvae move along the path 

 of the rays (Mast, '11, pp. 177, 178). This variability of re- 

 action is more apparent when the larvae aj"e subjected to sudden 

 changes in the direction from which the light operates. 



Without attempting to ascertain the relation of this variability 

 in sensitiveness to the age of the larvae, a method has been de- 

 vised by means of which larvae can be standardized as to their 

 reactivity to light (Patten, '14, '15). The test employed con- 

 sisted in subjecting larvae to an instantaneous change of 90° in 

 the direction of a horizontal beam of light. The abruptness with 

 which they came into orientation to the new direction of the rays, 

 was regarded as an index of their photoreactivity. In the experi- 

 ments referred to, this test was used merely to select, for further 



