HELIOTROPISM OF ANIMALS 83 
2. The heliotropic irritability of the oral pole of an ani- 
mal is different from the irritability of the aboral pole, and 
is generally greater than the heliotropic irritability of the 
aboral pole. 
3. The irritability of the ventral surface is different from 
the irritability of the dorsal surface. 
These three conditions taken together cause dorsiventral 
animals to place their median planes in the direction of the 
rays, and to move toward or away from the source of light in 
this direction. 
4, Eyeless animals (such as the larvee of Musca vomitoria) 
behave in this respect like animals having eyes. 
III. The heliotropic irritability of an animal manifests 
itself frequently only at certain epochs of its existence. 
1. In winged ants this epoch is the time of the nuptial 
flight. 
2. In plant lice it is the time when wings are present. 
3. In the larve of Musca vomitoria negative heliotropism 
is most prominent when they are fully grown. 
4, Ina large number of animals the sense of heliotropism 
is of the opposite kind in the larval and the adult states. 
5. Both the night and day Lepideptera are positively 
heliotropic, and their heliotropism is similar to that of every 
other positively heliotropic animal. The period of sleep of 
the night Lepidoptera, however, falls in the daytime, and 
only for this reason is their heliotropism manifested exclu- 
sively at night. 
IV. In many animals heliotropic irritability is connected 
with sexuality. Aside from the nuptial flight of ants, the 
fact must be mentioned here that in ants and Lepidoptera 
the males are heliotropically more sensitive than the females. 
V. The behavior of an animal depends on the sum total 
of its different forms of irritability. In this way it may 
happen that Cuma Rathkii and the caterpillars of the willow. 
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