VERMES, FLY LARVAE, AND ECHINOhERMS i8l 



mirror, i.e., toward the source of light, it soon begins to 

 crawl and turn so as to direct the anterior end away from 

 the mirror, but In attempting this the anterior end extends 

 into the direct light owing to the narrowness of the shadow. 

 This produces a stimulus which causes it to withdraw and 

 swing in the opposite direction, where it soon comes Into 

 the intense direct sunlight again. It continues this swing- 

 ing and crawling from one side of the shadow to the other 



Fig. 29, Representation of arrangement of apparatus used to produce light 

 conditions in which negative fly larvae crawl toward the source of light or perpen- 

 dicular to the rays, a, glass plate; b, b', beams of Hght; c, shadow cast by the 

 opaque standard d; e, fly larva; g, Nernst glower; s, opaque screen; m, m', mirrors, 

 relatively much farther from the glass plate than represented. The larva is placed 

 in the shadow, c, which is illuminated by light reflected from either the mirror m' 

 or m. The intense illumination on the anterior end whenever it projects beyond 

 the shadow prevents the larva from turning around and shows that under the 

 conditions of the experiment it is the change of light intensity on the anterior end, 

 and not the direction of the rays or the relation of intensity on symmetrically 

 located sensitive points, which regulates the direction of movement. 



for a short time, but soon comes to travel more nearly 

 parallel with the edges of the shadow and consequently 

 extends into the light much less frequently. If the rays 

 from the mirror are perpendicular to the edges the larva 

 also remains in the shadow, but usually It crawls along near 

 the edge farthest from the mirror. The negative larvae can 



