n 



n 



m 



Fig. 31. The process of locomotion and orientation in blow-fly larvae, a-j, 

 diflferent positions taken during the process; m, n, direction of light rays. The 

 anterior end of the larvae is quite regularly turned from right to left during the 

 process of locomotion. If n is exposed and m shaded simultaneously when a larva 

 is at d, it turns sharply to e, then loops to /, turns and expands to g, where the 

 sensitive anterior end becomes fully exposed and consequently stimulated. This 

 causes the larva to turn sharply at once to h, where it becomes attached and loops 

 to i, expands and turns toj and is again stimulated, after which it repeats its former 

 response, etc., until it is oriented and the oral end is no longer subjected to marked 

 changes of intensity, as it swings back and forth in the process of locomotion. If 

 n is exposed when the larva is in position h, no reaction takes place until it expands 

 and turns to c, then it responds as described above. If the light from n is much 

 more intense than that from m, or if the larva is in a very sensitive state it responds 

 at once when n is exposed no matter in which position it is. If it is at a or 6 it throws 

 the anterior end sharply toward n, then in the opposite direction, after which it 

 orients as described above. It may however wave the anterior end back and 

 forth several times before it orients. 193 



