172 LIGHT AND THE BEHAVIOR OF ORGANISMS 



condenser where it produced a sharply defined rectangular 

 area of intense Ught with a narrow triangular shadow pro- 

 jecting from one side. By manipulating the mirror I was 

 able to change the position of this area of light so as to 

 illuminate or shade any part of a larva fairly accurately in 

 spite of its microscopic size. All light except that from 

 the opening in front of the mantle was thoroughly elimi- 

 nated by means of suitable screens. Without going into 

 detail regarding the numerous observations made at dif- 

 ferent times, the redactions may be summarized as follows: 

 (a) If the anterior end is suddenly illuminated the larva 

 bends from side to side vigorously, (b) If the light inten- 

 sity on one eye-spot is increased without changing that on 

 the other, it bends both ends rather sharply toward the 

 illuminated side, (c) If the intensity on either eye-spot is 

 decreased it also bends toward the illu ninated side, (d) If 

 any portion back of the eye-spots is shaded or illuminated 

 there are no definite reactions. This shows that the ante- 

 rior end (probably the eye-spots) is the most sensitive part 

 of the larvae if it is not the only sensitive part, and that if 

 the light intensity is increased or decreased on either side 

 regardless of the direction of the rays, the larvae turn 

 toward the more highly illu ninated side. 



(2) Two sources of light were so arranged and screened 

 as to produce small horizontal beams which crossed at right 

 angles on the stage. The larvae exposed in the light from 

 these two beams oriented toward a point approximately 

 halfway between the two sources. If the light from one 

 source was now intercepted they turned directly toward the 

 other, and when it was again exposed they returned to their 

 former position. These reactions do not occur in all in- 

 stances nor is the orientation always precise and definite, 

 especially if the larvae are not in prime condition. They 

 were however seen in so many cases that there can be no 

 doubt concerning the conclusions stated above. 



What bearing have these conclusions on the problem in 

 hand? It is evident that when the light in one beam is 



