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. 1t 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 chrysorrheea, 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- 
Digitized by Microsoft® 
