56 STUDIES IN GENERAL PHYSIOLOGY 
tability exists in the two sexes of the Lepidoptera. Réaumur 
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 
Macheerites 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 OF THE LARVE OF 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 
15EMPER, Die natirlichen Existenzbedingungen der Thiere, Vol. I, p. 101. 
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