HELIOTROPISM OF ANIMALS 43 



behavior of an animal is merely the resultant of all its 

 forms of irritability, and so it may happen that an animal 

 is positively heliotropic even when it has no opportunity to 

 make use of it. The larvse of many saw-flies behave just as 

 the caterpillars of Lepidoptera. I have made observations 

 on the larvse of Nematus ventricosus, which are exactly like 

 those on Porthesia chrysorrhoea, which have been described. 



I have not yet succeeded in demonstrating a heliotropic 

 reaction to diffuse light in the indigenous pupae. Wilhelm 

 Mtiller, however, has observed effects of light in South 

 American species. 1 The pupa3 can move at three joints. 

 Only a lateral movement to the right and left is possible in 

 some of the species; in other species only a dorsal move- 

 ment of the body is possible; in a third species of pupae a 

 combination of both kinds of movements is possible. Miiller 

 observed that all three classes of movements can be brought 

 about under the influence of light. He found that some 

 pupse turned not only away from the light, but also toward 

 it. He also found that when the animals had been exposed 

 to the dark for some time, they "needed some time to become 

 susceptible again to the influence of light." In interpreting 

 the phenomena Mtiller follows the Darwinian idea, so that 

 the thought never occurs to him that he might be dealing 

 with phenomena similar to the heliotropic phenomena of 

 plants. 



The negative geotropism of the Lepidoptera. The 

 movements of very young or recently hatched animals have 

 for the most part been misunderstood, because they have 

 always been considered a function of mysterious "instincts" 

 of the animals, while the direction of their motions is in 

 reality determined by definite external forces. The same 

 cause which prescribes the course of a falling stone or deter- 

 mines the orbits of planets, namely gravitation, determines 



i MILLER, Zoologische Jahrbiicher, Vol. I (1886), pp. 568 ff. 



