256 BRADLEY M. PATTEN 



of intensity no longer occur. In both organisms, such a con- 

 dition is brought about by an ahgnment of the axis of the body 

 with the direction of the rays. In Stentor, when the ahgnment 

 takes place, the spiral turning no longer produces changes in the 

 amount of stimulus ; when it is attained in the blowfly larva, the 

 shadow of the posterior end of the body produces a shaded field 

 in which the swingings of the anterior end do not involve any 

 considerable changes in stimulation unless they are carried be- 

 yond the shadow, when the resulting motor reflex throws the 

 head back into the shadow again. 



Instead of orienting by the method just described, which is 

 perhaps the most characteristic, the larva may swing into align- 

 ment with the rays of light by a different method. If a larva is 

 suddenly subjected to a light from the side there is, in this second 

 method of orientation, no cessation of locomotion and no wig- 

 wagging. The anterior end simply swings a little more to the 

 side away from the light than it swings in the normal locomotor 

 movements, and the orientation is brought about graduall)^ 

 instead of immediately. Under the same conditions, one larva 

 may orient by one method and the next larva by the other method. 

 Moreover the same larva may use both methods of orientation. 

 In figure 2, c, the upper trail was made by a combination of the 

 two methods. When the light was turned on from the left, a 

 gradual bending of the trail appeared, due to wider turnings 

 away from the light than toward it. This was kept up to the 

 point at which the trail turns suddenly (fig. 2, c,x). Here the 

 larva stopped crawling, raised its anterior end, and performed 

 several wigwag movements; finally it fixed its anterior end 

 directly away from the light, and immediately came to a perfect 

 orientation. What the cause of the sudden change of tactics 

 was, I cannot say, but the sharpness of orientation at the place 

 where the wigwag movements were made, in contrast to the 

 gradual turning up to that point, is very apparent. The differ- 

 ence between these two methods of orientation, however, is not 

 fundamental. The greater extent of the swingings termed 

 'wigwag movements' naturally brings about orientation more 

 abruptly. In the case where the swinging is not so pronounced, 



