LIGHT AND THE MUSCLE TONUS OF INSECTS. THE 

 HELIOTROPIC MECHANISM. 



By WALTER E. GARREY. 



{From the Physiological Laboratory of Tulane University, New Orleans) 



(Received for publication, July 23, 1918.) 



The profound influence which Hght exerts upon the activities, 

 movements, and behavior of most insects, bespeaks its deep seated 

 effects upon the physiological processes within the neuromuscular 

 system of these forms. Our investigations have estabhshed a direct 

 relationship between the tonus or tension of the skeletal muscles and 

 the illumination of the eyes of heliotropic insects, and show that the 

 coordinated movements of these animals, like those of vertebrates, 

 are dependent upon the maintenance of an adequate and balanced 

 state of tonus in symmetrical groups of muscles. 



It was pointed out in a preceding communication on this subject, 

 that in the movements of heliotropic insects, the orientation was 

 determined by the changes in muscle tension induced by unequal 

 illumination of the two eyes.^ These results were in conformity with 

 the theory of hehotropism proposed by Loeb in 1888,^ and restated by 

 him as follows: ''The mass of photochemical reaction products fonned 

 in the retina .... influences the central nervous system and 

 through this the tension .... of themuscles. If the rate of photo- 

 chemical reaction is equal in both eyes this effect on the symmet- 

 rical muscles is equal; .... as a consequence the animal will not 

 be deviated from the direction in which it was moving. This happens 

 when the axis or plane of symmetry of the animal goes through the 

 source of light, provided only one source of light be present. If, 

 however, the light falls sidewise upon the animal, the rate of photo- 



1 Garrey, W. E., Proc. Nat. Acad. Sc, 1917, iii, 602. 



^ Loeb, J., Sitzungsber. physik.-med. Ges., Wiirzburg, 1888;Der Heliotropismus 

 der Tiere und seine Uebereinstimmung mit dem Heliotropismus der Pflanzen, 

 Wiirzburg, 1890. 



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