HELIOTROPISM OF ANIMALS 87 



the direction of the vertical and turn their heads upward. 

 There are, however, also animals which orient themselves in 

 exactly the reverse manner and turn their heads downward. 

 To this class belongs the garden spider, which we find hang- 

 ing in this position in the center of its web for hours. I 

 found the same behavior in one of the Diptera, which I have 

 not yet classified. 



5. If a beetle or a house fly (from which the wings have 

 v Jbeen removed) is placed on the disk of a centrifugal machine, 



which is rotated slowly, the insect turns its body around the 

 same axis, but in an opposite direction to that of the revolv- 

 ing plate. 



If the velocity of the machine is increased, these compen- 

 satory movements cease: These animals therefore behave in 

 this respect exactly as Mach claims vertebrates behave which 

 possess a labyrinth. 1 But while the movements of vertebrates 

 continue for some time after the movement of the centrifugal 

 machine has ceased, but in a sense opposite to those occur- 

 ring during rotation, I have never been able to bring about 

 these compensatory after-movements in insects. 2 



6. When one hemisphere of the brain of a house fly is 

 removed the same disturbances in orientation appear as after 

 the same operation on a rabbit. The fly from which the left 

 hemisphere has been removed moves continually toward the 

 right in its progressive movements. These deviations are 

 greater or less according to the success of the operation. I 

 showed in an earlier paper that the turn-table reactions of 

 dogs and rabbits deprived of a hemisphere might be unsym- 

 metrical. If a fly which has lost the left half of its brain is 

 placed on a rotating disk, we find that on turning the disk 

 in the direction of the hands of a watch as seen from above, 



1 MACH, Grundlinien der Lehre von den Bewegungsempfindungen (Leipzig, 1875). 



2 The observations of Lyon and of Radl suggest the possibility that these phe- 

 nomena depend upon the eyes of these animals. When the eyes are removed or black- 

 ened the reactions cease. [1903] 



