HELIOTROPISM OF ANIMALS 19 



responsible for the heliotropic phenomena which take place 

 so energetically behind this screen, but can be due only to 

 the activity of the more refrangible rays. 



The other external conditions which must be considered 

 in heliotropic investigations are so simple that they do not 

 call for any special explanations. Where they are of impor- 

 tance they will be self-evident. 



It is very essential, however, to realize that the helio- 

 tropism of an animal often manifests itself clearly only dur- 

 ing a definite, often decisive, period of its existence, only to 

 diminish again or to disappear entirely later. It was only 

 by observing for weeks and months the animals described in 

 this treatise, which for the most part I raised myself, that 

 I have been able to establish this fact. 



The caterpillars of Porthesia chrysorrhoea, for example, 

 are energetically positively heliotropic only during a certain 

 period of their existence, when they have just left the coc- 

 coon in which they have wintered, and have not yet taken 

 food. At this time the entire existence of these animals is 

 a function of the light. Under natural conditions they 

 hatch out on a warm spring day. The light compels them 

 to creep ' to the tips of the branches, where they find their 

 first nourishment in the young buds. When fed they are 

 still positively heliotropic, but very much less so than before. 

 If anyone should examine them in this condition, he would 

 scarcely pronounce them heliotropic. 



It is not, however, a certain date of the year which gov- 

 erns this heliotropism ; for whenever I forced the animals to 

 leave their nest (by raising the temperature), whether at the 

 beginning of summer or of winter, they were indefatigable 

 in their attempts at creeping toward the source of light. 



Winged ants are pronouncedly dependent on light only 

 at a definite period of their existence at the time of their 

 nuptial flight. The same animals which were actively helio- 



