QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 261 



response in an animal which was normally indifferent to light, as 

 recorded by Moore ('13). 



The cases of natural and artificial asymmetry of suscepti- 

 bility to light are not the only indications that the light recep- 

 tive system operates on the musculature bilaterally. The 

 deflection of symmetrically responding larvae toward the weaker 

 light is in every way comparable with the deflection of asym- 

 metrical larvae toward their less sensitive side when the illumi- 

 nation of both sides is equal. In one case the unbalanced factor 

 of the response is external and in the other case it is internal. 

 Moreover, it is possible to balance the internal asymmetry of the 

 photosensitive areas by subjecting them to a corresponding in- 

 equality of illumination from without. For example, suppose, 

 when a larva is subjected to equal lights from opposite sides, it 

 deflects 5° toward the left. By referring to the curve of figure 15, 

 it will be seen that, with asymmetry balanced out, a deflection 

 of 5° is produced by a difference of 15 per cent between the 

 opposed lights. With the left-hand light 15 per cent stronger 

 than the right, the same larva crawls straight down between the 

 lights like a symmetrically responsive larva crawling under the 

 influence of equal lights. 



The physiological asymmetry has been corrected by a difference 

 of 15 per cent in the bilateral stimulus. Thus we can not only 

 demonstrate that certain individuals do not have a perfect bi- 

 lateral balance of sensitiveness but we can measure the amount of 

 the asymmetry and correct it by applying bilateral light stimuli 

 of a corresponding difference in intensity. 



The evidence that sjth metrically located sensitive areas 

 operate bilaterally on the musculature may be summarized as 

 follows: (1) When the lights acting on the opposite sides of the 

 larva are equal, the larva orients so that its median plane is 

 at right angles to the line connecting the sources of light. (2) 

 When the opposing lights are unequal, a deflection toward the 

 weaker light appears. (3) Certain larvae are asymmetrical 

 in their response, deflecting toward the less sensitive side when 

 subjected to equal bilateral illumination. (4) The blackening of 

 one side of the sensitive region produces a deflection toward 



