276 BRADLEY M. PATTEN 



definite axial orientation with reference to light, the tropism may 

 be regarded as a special case of phototaxis in which the response 

 depends on bilaterally placed sensitive areas. 



The analysis of the factors involved in orientation as it has 

 been presented here has been stripped of many details and limi- 

 tations because I believe it has more value when stated broadly 

 enough to allow details to be added as our knowledge of the sub- 

 ject increases. 



I wish to express my indebtedness to Prof. G. H. Parker, 

 xmder whose supervision this work was done, for his interest and 

 helpful criticism, and to Dr. E. L. Mark for the facilities of the 

 Zoological Laboratory and for the many courtesies shown me 

 during my work there. I also wish to express the deepest grati- 

 tude to my father, who first aroused my interest in biology and 

 whose criticism and encouragement have been a constant help. 



SUMMARY AND CONCLUSIONS 



1. The wide range of individual variability in the response of 

 the blowfly larva to light renders a study based on untested ani- 

 mals of little value as a basis for comparing the effects of different 

 experimental conditions. 



2. To obtain reliable data for a comparative study of light re- 

 actions, it is necessary to establish a 'standardization test' and to 

 make use of those individuals only, that show a uniform degree of 

 sensitiveness. 



3. To measure accurately the reaction to light in terms of 

 definite physical units, the larvae have been subjected to the 

 simultaneous action of opposed horizontal beams of light of 

 known intensity, and the response measured, in degrees, on the 

 resulting angular deflection of the trail. 



4. When the opposing lights were of equal intensity, the aver- 

 age trail of the standardized larvae was within 0.09° of the per- 

 pendicular to the line connecting the sources of light. 



5. When the opposing lights were unequal, the 'average trail' 

 showed a deflection toward the weaker light. The amount of the 



