56 STUDIES IN GENERAL PHYSIOLOGY 



tability exists in the two sexes of the Lepidoptera. Reaumur 

 states that in the main only males fly into the candle flame. 

 From this fact, which is correct, it follows that it must 

 require a more intense light to cause the females to execute 

 heliotropic movements than is necessary for the males. Both 

 male and female moths are attracted by sources of light 

 which are stronger than the candle flame, for instance, the 

 electric arc light. It is a well-known fact that the females 

 fly less than the males. It is imaginable that this is due to 

 the fact that the females are less irritable toward the light 

 than the males. 



The difference in the irritability of male and female ants 

 toward light brings up the question as to whether the differ- 

 ence in the development of the sense organs, particularly 

 the eyes, which is often observed in males and females of 

 the same species, is connected with this difference in irrita- 

 bility. The males of ants have larger eyes than the females. 

 But the cause of the difference in sensitiveness may lie 

 deeper, as is, for example, indicated by the following obser- 

 vation made by Semper : "In all species of the cave beetle 

 Machserites only the females are blind, while the males have 

 well-developed eyes ; notwithstanding this fact they always 

 live together." ' Eyes therefore develop more easily in males 

 than in females even in the dark. It might be worth while 

 to determine whether in these cave-dwellers the males are 

 also heliotropically more sensitive than the females. 



IX. THE NEGATIVE HELIOTROPISM AND OTHER FORMS OF 

 IRRITABILITY OP THE LARVJ2 OP MUSCA VOMITORIA 



The phenomena of irritability in negatively heliotropic 

 animals obey the same laws as those in positively heliotropic 

 animals ; with this difference, however, that negatively helio- 

 tropic animals turn their aboral poles toward the source of 

 light instead of their oral poles, and that in consequence the 



1 SEMPER, Die natilrlichen Existenzbedingungen der Thiere, Vol. I p. 101. 



