116 HELIOTROPIC MECHANISM 



When these insects move they do so in a most clumsy incoordinated 

 manner, always in small circles to the right in spite of the fact that 

 equal areas of the two eyes are blinded, indicating again the fact 

 that different areas of the eyes control the tonus of entirely different 

 groups of muscles. Flying movements are absolutely disorganized. 

 The wings have no hf ting power and the fly simply spins about a verti- 

 cal axis, never leaving the surface of the table. If thrown into the 

 air, the body may rotate on its long axis and gyrate in so incoordi- 

 nated a way that schematic analysis is impossible. One is reminded 

 again of asymmetrical lesions in the labyrinths of the vertebrate ears. 



Effect of Symmetrical Blackening of Eyes: — Symmetrical blackening 

 of the eyes, both outer halves or both inner halves for example, 

 does not result in any asymmetry of posture either of the body, legs, 

 or wings of Proctacanthus, provided equal areas are blackened on the 

 two sides. There is weakening of some groups of muscles, but, the 

 two sides being symmetrically affected, there are no forced motions 

 either to the right or left. 



Comparison of the Insect's Eye with the Otic Labyrinth of Verte- 

 brates. — The experimental results afford irrefutable proof that the 

 tonus of the muscles, their excitability, and force of contraction, 

 and thus the basal conditions for coordinated movement, are depend- 

 ent upon the effects of light upon the eyes of heliotropic insects. The 

 otic labyrinth subserves these functions in vertebrates. In the 

 latter, entirely different groups of muscles are affected by different 

 parts of the labyrinth, and the analogy to the insect's eye is made 

 complete by our demonstration that different parts of the eye control 

 the tonus of different groups of muscles. Blackening the right eye 

 affected very different groups of muscles from those affected by a 

 similar treatment of the left eye ; hence the circus motions were to the 

 left or to the right respectively. The outer and the inner halves con- 

 trol different muscles. There are striking differences between the 

 effects of blackening the upper and the lower halves of the eyes. 

 There thus exists an optical mosaic in which the receptors are arranged 

 in definite patterns which are mirrored in the groups of muscles in- 

 volved in the effects of light and shade on these different areas. 



It is possible even to trace, with a considerable degree of certainty, 

 the path of the nerve impulses from the eyes to the different groups of 



