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 larve of many saw-flies behave just as 
the caterpillars of Lepidoptera. I have made observations 
on the larve of Nematus ventricosus, which are exactly like 
those on Porthesia chrysorrhcea, which have been described. 
I have not yet succeeded in demonstrating a _heliotropic 
reaction to diffuse light in the indigenous pupe. Wilhelm 
Miller, however, has observed effects of light in South 
American species." The pupx 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 pups a 
combination of both kinds of movements is possible. Miller 
observed that all three classes of movements can be brought 
about under the influence of light. He found that some 
pup 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 Miller 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 
1 MULLER, Zoologische Jahrbicher, Vol. I (1886), pp. 568 ff. 
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