WALTER E. GARREY 125 



depending upon the speed employed. The brighter the Hght, the 

 easier it is to narrow the circles by rotation toward the blackened eye 

 and the more difficult it is to neutralize the forced motion by rotation 

 toward the normal eye. Possibly the similar behavior, on the turn- 

 table, of flies from which Loeb had removed one-half of the brain, was 

 due to the fact that the light stimulus from one side was lacking. 



In conclusion it may be stated that the condition of muscle tonus 

 produced by blackening the eyes was not recovered from in the weeks 

 during which many robber flies and butterflies were kept in the labor- 

 atory; as a matter of fact, the asymmetry of the muscles gradually 

 became more pronounced and was fixed after death by the onset of 

 rigor mortis. 



SUMMARY. 



The tonus of the muscles of heliotropic insects is due chiefly to the 

 action of light; it is markedly decreased in the dark. Each eye con- 

 trols the tonus of a difterent group of muscles on both sides of the body. 

 Different areas of each eye likewise are related to the tonus of different 

 muscle groups, and the relationship is entirely analogous to that of 

 the otic labyrinth of vertebrates. Asymmetrical conditions of 

 muscle tension are produced by any procedure which establishes an 

 unequal photochemical reaction in the two eyes, by difference in 

 illumination, by partial or complete blackening of one eye, or estab- 

 lishing unequal sensitiveness in the two eyes. The unbalanced con- 

 dition of muscle tonus expresses itself in unusual postures of the rest- 

 ing insects, and in movements in forced paths — circus motions when 

 one eye has been blackened. These reactions vary directly with the 

 intensity of the illumination, as shown not only by the variation in 

 diameters of the circles, but also by the reactions of the insects on 

 vertical surfaces and on the turntable. 



The relation of the results of these experiments to the problem of 

 heliotropic orientation is too obvious to require detailed discussion, 

 which could only lead to a repetition of the description of the mechan- 

 ism of hehotropism which Loeb has so clearly expounded. The 

 experiments are so completely in accordance with Loeb's muscle 

 tension theory of heliotropism, that they are tantamount to a com- 

 plete proof of it. 



