68 STUDIES IN GENERAL PHYSIOLOGY 



substance must be formed in the decomposition of the pro- 

 teids which attracts the Musca larvae even from a distance. 



The contact-irritability of Musca larva'. It is a well- 

 known fact that Musca larvae are inclined to crowd into 

 cracks and crevices in the earth, and it is astonishing 

 through what small cracks the adult larvae can slip. This 

 irritability might impress a Darwinian as though the ani- 

 mals wished to protect themselves from the light. That this 

 contact-irritability is entirely independent of their helio- 

 tropism is shown by the fact that these animals crowd them- 

 selves under a completely transparent glass plate, even if by 

 so doing they have to move toward the light. 



The animals retain this form of irritability even when put 

 into a vessel of water, in which they soon die. I noticed 

 this phenomenon in feeding tritons with fly larvae. Small 

 stones lay on the bottom of the vessel, and the larvae crowded 

 themselves under them as eagerly and as skilfully as if 

 they had always lived under them. The perniciousness of 

 this irritability in the case in question is apparent when we 

 remember that it keeps the animals from reaching the sur- 

 face of the water again, so that they are drowned. 



In these experiments I was struck by the fact that the 

 animals, when placed under the surface of the water, do not 

 swim upward and so avoid death, but swim downward. I 

 cannot explain this fact. Under other conditions positive 

 geotropism cannot be demonstrated in these animals. 



The posit ire heliotropism of flics at the time of sexual 

 maturity. The fly, which as a larva is negatively helio- 

 tropic, is positively heliotropic in the state of sexual 

 maturity. This reversal in the sense of heliotropism in 

 changing to the adult state is not uncommon. Yet it is a 

 striking fact that, while heliotropism is reversed, the orienta- 

 tion toward chemical substances is the same in the female 

 flies at sexual maturity as in the larval state. 



