228 BRADLEY M. PATTEN 



in the trails shown in figure 3, a, b and c, these trails do not come 

 to lie parallel to the new ray direction. These larvae were sen- 

 sitive to the light but not sufficiently so to orient accurately and 

 therefore were not used in making the ^-ecords on which the tables 

 are based. 



In testing specimens from cultures at the optimum stage, from 

 15 to 25 per cent of the larvae were discarded because they failed 

 to orient sharply to the change of direction in the light. Al- 

 though the maggots from the same cultures were all of the same 

 age, they were not necessarily in the same stage of development. 

 The discarded larvae probably represented individuals that had 

 passed, or not yet reached, their period of maximum responsive- 

 ness, rather than* those which would never reach the normal 

 degree of sensitiveness. 



The larvae showing the desired accuracy of orientation were 

 laid aside in numbered boxes and, after a rest, subjected to 

 bilateral illumination from the mirror beams. Such a selec- 

 tion gave uniformity to the animals used in compiling the final 

 results, and though it raised their standard of sensitiveness 

 somewhat above that of the general population, it did not in any 

 way distort the relative values of the subsequent records; on the 

 contrary it is only by thus insuring uniformity in the material 

 used that the comparison of results from different larvae under 

 various experimental conditions becomes of value. 



4- Plan of , experiments 



The plan of the experiments was to subject larvae of standard 

 sensitiveness to lights of accurately determined intensity, with 

 the conditions so arranged that the reactions could be measured 

 in physical units. The rate of locomotion of the larva under the 

 influence of light of various intensities has proved a difficult and 

 unsatisfactory basis for a quantitative study (Mast '11, pp. 

 184-189). But orientation is a phase of the response to hght 

 which lends itself admirably to accurate measurement. 



According to Loeb ('05, p. 2) : "If two sources of light of equal 

 intensity and distance act simultaneously upon a heliotropic 



