HELIOTROPISM OF ANIMALS 51 



collected at the highest point in the test-tube. This experi- 

 ment must, of course, be made in a dark room. When the 

 animals are first brought into the dark, the experiment can 

 be repeated many times with exactly the same result ; every 

 change in the position of the test-tube with reference to the 

 vertical compels the animals to creep upward and to collect 

 at the highest point in the tube. When, however, the ani- 

 mals were kept permanently in the dark, the reaction ceased 

 soon, and the animals remained motionless, no matter how 

 often the position of the test-tube was reversed. The ani- 

 mals were in a sort of rigor. When they were placed on an 

 inclined or vertical plane, they moved upward. Geotropic 

 orientation occurred as soon as the plane made an angle of 

 30 with the horizontal; the geotropic movements were the 

 more certain and precise the nearer the plane approached the 

 vertical. When light fell on the animals at the same time, 

 their orientation was determined by the resultant of the 

 direction of the rays of light and gravitation, in which, how- 

 ever, the light was the stronger force even at a great distance 

 from the window. 



The winged animals behave toward a source of heat in the 

 same manner as the caterpillars of Porthesia chrysorrhoea. 

 When I brought the animals in an opaque vessel into a room 

 having a temperature of 18 and placed them near a stove, 

 they left the side of the vessel which was turned toward the 

 stove, as soon as its temperature increased a few degrees. At 

 a temperature of 9 the animals were so sluggish that a 

 definite reaction to light or gravity did not take place. A 

 temperature of 20-24 is the most suitable for the experi- 

 ments. When I surrounded one-half of the vessel with a 

 water-bag having a temperature of 20, the other half with 

 one having a temperature of 10? 5 the animals moved, under 

 the influence of light, from the warmer into the cooler area. 

 But they did not move far into the latter, as their movements 



