HELIOTROPISM OF ANIMALS 53 



the direct sunlight into the shade. The direction of the rays, 

 and not the distribution of the intensity of the light, in the 

 test-tube, therefore, determines the direction of the pro- 

 gressive movements. 



The blue rays were pre-eminently effective. When the 

 test-tube was covered with blue glass, either entirely or in 

 part, the orientation was changed in no way. When the 

 tube was entirely covered with red glass,* the movements 

 occurred more slowly. The animals finally collected on the 

 window side, but it took a long time. When the tube lay 

 with the longitudinal axis perpendicular to the window, and 

 the portion nearest the window was covered with red glass, 

 the animals collected at the boundary between the uncovered 

 and covered parts. Diffuse daylight affected the animals 

 just like sunlight. These facts may suffice to show that at the 

 time of the nuptial flight the winged ants are energetically 

 positively heliotropic. 



Yet I found that up to the time of the nuptial flight, 

 light had practically no effect on winged ants which were 

 taken from the same nest. 



Animals which I collected after the nuptial flight also did 

 not react very distinctly to light. If heliotropism was still 

 present at all, it was obscured by other forms of irritability, 

 particularly stereotropism. 



The nuptial flight of the ants of this nest always took 

 place at about 5 o'clock in the afternoon, when the sun's 

 rays fell upon the nest. That it was the latter condition, 

 and not the time of day, which determined the period of 

 flight is shown by the fact that in other nests, which were 

 reached by the sunlight earlier in the day, the flights took 

 place earlier. Usually the flight occurs at about noon, when 

 the sun's rays strike the earth perpendicularly and the tem- 

 perature is relatively high. Both the males and the females 

 which I collected from the swarm which had left the nest 



