HELIOTROPISM OF ANIMALS 83 



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 lame 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 larvse of Musca vomitoria negative heliotropism 

 is most prominent when they are fully grown. 



4. In a 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 Lepidoptera 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- 

 sivelv at night. 



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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- 



