44^ 'Journal of Comparative Neurology and Psychology. 



results as the above can be obtained more simply by placing the 

 jar directly in the sunlight. These experiments also included the 

 use of blue and red glass to transmit the rays. Red glass gave the 

 same effect as darkness. The blue gave normal results. 



Experiment 6. — The experiment was next tried with an aqua- 

 rium of special form to show both the downward movement and 

 the positive movement horizontally toward the light. For this 

 purpose a deeper aquarium was used, two feet in depth, with the 

 other dimensions six by twelve inches. The narrow ends were of 

 glass. The rest of the interior was blackened. It was placed in 

 the direct sunlight with the light entering from one end and the 

 top covered. The larvae moved downward and toward the lighted 

 end, collecting in the lower lighted corner. When large numbers 

 are used the result is very striking. Not all collect in this way, as 

 a few will be scattered through the aquarium. With freshly col- 

 lected animals the movements are rapid and appear hurrying in 

 spite of the discontinuity. When the aquarium is turned end for 

 end they begin to move in the other direction with great quickness. 

 These experiments indicate that when the water is illuminated 

 from whatever direction the larvae follow their geotropic instinct 

 to go downward. In darkness or weak light they are negatively 

 geotropic or positively phototropic to weak light. 



The positive response to strong light offers some difficulty 

 since it would naturally be expected to cause the animals to go up 

 as well as horizontally. The pupae do move to the region of great- 

 est intensity, up or down or to the side. In the larvae the geotro- 

 pism is the stronger. The musculature involved in downward 

 movements is different in part from that employed in horizontal 

 movements. The positive reaction changes to negative in from 

 one to two hours or in some collections in less time. It should 

 be observed however that the gathering on the lower lighted side 

 of the aquarium precludes the explanation that the downward 

 movement is due to a negative light reaction. 



The case of Corethra perhaps presents some similarity to the 

 nocturnal amphipods described by Holmes, which were positively 

 phototropic. Parker ('oi) ascribes the downward movement 

 of Copepods in the daytime partly to negative phototropism. As 

 shown by Experiment 3, the case of Corethra cannot be due to 

 negative phototropism, although the depth migrations are not inde- 

 pendent of light. 



