88 STUDIES IN GENERAL PHYSIOLOGY 
the fly makes slight or only weak compensatory movements 
Yet when the disk is turned in the opposite direction the fly 
reacts very promptly (apparently even better than before the 
operation). After destroying both hemispheres or ampu- 
tating the head I no longer obtained compensatory move- 
ments in the fly. The experiments of Mach show that the 
compensatory movements of vertebrates emanate from the 
head ; according to Mach, the labyrinth is to be considered 
the essential organ. Such an organ, however, does not 
exist in the head of a fly. 
7. If the halteres are removed from a fly, it can no longer 
fly upward ; in the attempt to fly it immediately falls to the 
ground, where it frequently tumbles about. Gleichen- 
Russwurm established this fact during the last century. I 
found that such a fly reacts normally on the centrifugal 
machine. The destruction of the halteres does not there- 
fore have the same effect as the destruction of the labyrinth 
in frogs, birds, or mammals, in which, according to the 
experiments of Hégies and Schrader, compensatory move- 
ments cease when the labyrinth is destroyed. The conjec- 
ture expressed by others, and by me in my first publication, 
that the direction of sound has an influence on orientation 
has thus far led me to no new facts. 
8. I have not yet been able to demonstrate compensatory 
movements on the centrifugal machine in caterpillars, Musca 
larvee, and snails. 
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