210 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



reactions, nevertheless do not eliminate the possibility of a response to 

 the intensity of the light. These conditions were secured by the use 

 of the Y-tubes, already mentioned. The experiments therewith 

 serve to demonstrate why the tendency to gather in the brighter 

 areas (assumed positive photopathy) is usually associated with 

 positive phototaxis; and why a tendency to gather in the darker 

 areas (assumed negative photopathy) is usually associated with 

 negative phototaxis. The citation of a few experiments will serve to 

 illustrate both technique and results. 



In the diagrams of the accompanying plate are represented the 

 Y-tubes as set up for experiment. Those whose horns are above 

 were arranged for experiment with larvae having positive phototactic 

 reaction; those whose horns are below, for larvae having a negative 

 phototactic reaction. In tubes A and B, one side of one horn (c) was 

 fitted with a band of black paper which extended half over the cir- 

 cumference of the horn, and reached a very short distance down each 

 stem. In tubes C and D the same arrangement existed, save that 

 white instead of black paper was used. In every case the light rays 

 came from the window in the direction indicated by the arrows. In 

 all cases of larvie manifesting a negative reaction, the start was made 

 at the end of the tube (lying horizontally on the table) nearer the 

 window. In the case of positively reacting larvae, on the other hand, 

 the start was made from the end of the tul)e farther from the window. 

 The end designated a, was in every instance the end frojn which the 

 larvae moved. The point of the experiment was to determine in 

 which horn of the Y-tube the larvae would eventually congregate. 



Case 1. 



In this instance the tube was arranged as in Fig. A. Ten posi- 

 tively reacting, first-stage larvae were placed in the Y-tube and, 

 by certain manipulations of the light and by virtue of their positive 

 reaction, were made to congregate in region a. Then suddenly the 

 direction of the light was changed so as to come in the direction of 



