CHANGES OF PHOTOSENSITIVITY WITH AGE 



587 



experiments, larvae of uniform ' sensitiveness. The variability 

 in the photosensitivity of different individuals, made apparent 

 by it, indicated that a test of the same nature would serve to 

 show very accurately any changes taking place with advancing 

 age in the larva's reactivity to light. 



For carrying out this test three 220 volt, 25 watt Mazda lamps 

 were used. A light-proof case was made for each bulb. In one 

 face of each case was cut a horizontal rectangular aperture, 1.0 



Fig. 1 Diagram to show arrangement of lights used in testing the blowfly's 

 larva's photosensitivity. For explanation see text. 



X 3.5 cms. The lights were then fixed to the top of a table, 

 equidistant (23 cms.) from its central point; figure 1 shows dia- 

 grammatically their arrangement. Two of the lights (A and B, 

 fig. 1) were directly opposed, their central rays meeting in the 

 same straight line but from opposite directions. These two 

 lights we may call the ' starting lights.' The third light (fig. 1, C) 

 was so placed that its central ray intersected the line connecting 

 the starting lights perpendicularly at its midpoint. 



Under the influence of one of the starting lights (the other 

 lights being shut off), larvae were made to crawl until they 



