Walter e. garrey 111 



The fact that flies which have always been bHnd in one eye show the 

 same changes in the muscle tonus and the same motor reactions as 

 other insects with one eye blackened, eliminates all idea of "avoiding 

 reactions." The further fact that the behavior of the insects in the 

 above experiments, and in those to be described, is due to forced 

 motions dependent upon the conditions of muscle tonus which exist 

 in the resting animals, also disposes of any relation to "trial and 

 error." 



We may summarize the results of the above experiments by stating 

 that the tonus of the muscles depends upon the illumination of the 

 eyes. The muscle groups affected by the right eye are different from 

 those affected by the left eye, thus a difference in illumination of the 

 two eyes, whether induced by illuminating one or blackening the 

 other, produces an asymmetry when the insects are at rest. The 

 conditions of muscle tonus thus produced are such that if the animals 

 move, they are forced toward the side of the greater illumination. 

 As previously pointed out,^ the mechanism of this forced motion is the 

 same as that in the movement of animals in which the tonus of 

 the muscles has been produced by other means, such as passing the 

 electric current through the body as described by Loeb and Maxwell" 

 and by Loeb and Garrey.^^ 



Similar tonus changes and forced motions can also be produced by 

 ablation or defects in the internal ear of vertebrates, so that as an 

 organ of muscle tonus, the eye of insects is comparable to the otic 

 labyrinth of vertebrates. In elaborating this conception we shall 

 show that there are very intimate functional analogies between the 

 eye of the insect and the vertebrate ear, which emphasize the simi- 

 larity of this relationship to muscle tonus; thus the tonus of specific 

 groups of muscles is controlled by definite areas on the insects' eyes 

 much as they are in vertebrates by different parts of the otic laby- 

 rinth. The following experiments will make this clear. 



Effect of Blackening Part of One Eye. — This procedure has the effect, 

 although to a lesser degree, of blackening the whole eye; more accu- 

 rately, it has the effect of decreasing the illumination in that eye. 

 The extent of the tonus changes in the muscles is, roughly speaking, 



-3 Loeb, J., and Maxwell, S. S., Arch. ges. Physiol., 1896, Ixiii, 121. 

 -^ Loeb, J., and Garrey, W. E., Arch. ges. Physiol., 1897, Ixv, 41. 



