Tropisms and Instincts as Adaptations 389 
Butterflies that have just emerged from their pupa case 
exhibit a marked negative geotropic reaction, and this ap- 
pears to be connected with the necessity of unfolding their 
wings at this time. Loeb says that the same cause that 
determines the direction of the falling stone and the paths of 
the planets, namely, gravity, also directs the actions of the 
butterfly that has just left its pupa case. The geotropic 
response is especially strong at first. The animal wanders 
around until it reaches a vertical wall, which it immediately 
ascends, straight upward, and remains hanging at the top 
until its wings have unfolded. A similar response occurs in 
the final stage of the larva of the May-fly, which leaves the 
water and crawls up a blade of grass, or other vertical sup- 
port, and there, bursting the pupa skin, it dries its wings and 
flies away. That this is a reaction to gravity and not to 
light is shown by Loeb’s observation, that their empty skins 
are sometimes observed under a bridge where the light does 
not come from above. “This observation on the larva of 
the May-fly contradicts the assumption that the ‘purpose’ of 
the geotropic response of the butterfly is that it may the 
better unfold its new wings, for in the ephemerid larva the 
negative geotropism appears at a time when no wings are 
present.” On the other hand, it should not be overlooked 
that the reaction is important for the May-fly larva in other 
ways, because it leads the larva to leave the water at the 
right period, and come out into the air, where the flying 
insect can more safely emerge. 
It is not without interest to find that caterpillars ex- 
hibit some of the same reaction shown by butterflies. Loeb 
has made numerous experiments with the caterpillars of 
Porthesia chrysorrhea. The caterpillars of this moth col- 
lect together in the autumn and spin a web or nest in 
which they pass the winter. If they are taken from the 
nest and brought into a warm room, they will orientate 
themselves to the light, and also crawl toward it. If 
