104 HELIOTROPIC MECHANISM 



berg's sign in tabes dorsalis is due simply to a loss of muscle tonus 

 when ocular illumination is prevented rather than to true vision; 

 if so, it falls into the same category with the loss of tonus resulting 

 from the degeneration of the posterior columns whereby the afferent 

 impulses from the muscles and tendons fail. 



Ewald, Lee, Lyon, Luciani and others have shown that the muscle 

 tonus of vertebrates is maintained chiefly through afferent impulses 

 from different parts of the otic labyrinth. Ewald states that loss of 

 these impulses, by ablation or defect of the labyrinth, causes ''an 

 abnormal relaxation of the affected muscles, diminished energy during 

 activity, and diminished precision of the movements in which they 

 are concerned. "^^ The insecta show no lack of muscular tone or power 

 of precision or coordination of movements, although they possess 

 neither otic labyrinth nor structures which suggest that they might 

 function in any similar way. It is the intention of this report to show 

 that the reflex effects produced by illumination of the eye of helio- 

 tropic insects establish physiological conditions in their muscles 

 which are in every way analogous to these effects produced through 

 the internal ear in vertebrates. The initial experiments which sug- 

 gest this analogy were made by RadP^ but the observations of Holmes'-* 

 (1905) had a much more direct bearing upon the problem in hand. 

 He worked with the water scorpion, Ranatra, which is positively 

 hehotropic; it seems to have been especially favorable material since 

 both the descriptions and drawings of Holmes show striking changes 

 of posture produced by illuminating the animal from different direc- 

 tions, and by blackening the eyes. Our investigations show that 

 similar postures assumed by insects are the result of changes in the 

 tonus of various muscle groups due to the unequal or asymmetrical 

 illumination of the two eyes, a feature which is of general application 

 to a great many groups of the insects. 



Our experiments were conducted mainly upon the robber flies 

 which are possessed of long legs and powerful muscles. Proctacanthus , 

 (Fig. 1) was especially good material for postural studies, Promachus 



^® Ewald, J. R., Physiologische Untersuchungen iiber das Endorgan des Nervus 

 octavus, Wiesbaden, 1892. 

 i7Radl,'2 p. 52. 

 18 Holmes, /. Comp. Neurol, and Psychol., 1905, xv, 305. 



