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 chrysorrhasa. 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 



