HELIOTROPISM OF ANIMALS 538 
the direct sunlight into theshade. The direction of the rays, 
and not the distribution of the intensity of the light, tn 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 
-oecurred 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 
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