118 HELIOTROPIC MECHANISM 



takes place toward the unblackened eye, and butterflies which may not 

 lean toward the good eye, topple over toward that side if stimulated; 

 they resist pressure against the wings from the side of the good eye 

 but fall sideways if pushed or blown against from the side of the black- 

 ened eye. In ahghting from flight such butterflies topple over toward 

 the good eye as a result of the incoordination which results when 

 the leg muscles are suddenly called into action. Such butterflies 

 in righting themselves may spin about several times toward the 

 functioning eye. although when at rest they may show no postural 

 abnormahty. These reactions demonstrate that there exists a real 

 asymmetry in the physiological state of the muscles of the two sides, a 

 weakness and absence of tonus which is unmasked only when a moto r 

 effort is made. We have here a perfect analog}- to phenomena seen 

 in vertebrates with certain labyrinthine abnormahties. 



Asymmetrical Sensitiveness. — The maintenance of symmetrical 

 equilibrium in the tonus and physiological state of the muscles of the 

 two sides of the insects with which we worked, depends upon equiva- 

 lent stimulation of both eyes. This is accomplished if the intensity 

 of the illumination is the same, and the sensitiveness of the two eyes 

 is equal. The effect of a stimulus varies directly with its intensity and 

 also with the sensitiveness of the receptive tissue. It occurred to the 

 writer that circus motions should result if one eye could be made more 

 sensitive than the other; these should be in the direction of the more 

 sensitive eye when both are equally illuminated. It was noted that 

 Proctacanthus with one eye blackened shows the most pronounced 

 postural changes and moves in circles of smallest diameter when 

 brought from the dark room or during the early morning. The fact 

 was estabUshed that exposure to light produces considerable fatigue 

 of the eye and consequent weakening of the effect of light as noted in 

 the progressive increase in the diameter of their circus movements, 

 and that complete recovery of sensitiveness occurs after continement 

 in the dark. These facts suggested the expedient of simply removing 

 the black from one eye after exposing the other to light for some 

 hours. We were certain that the covered eye would be dark- 

 adapted and relatively much more sensitive than the eye which had 

 been exposed to light. Asphalt varnish hardens into a thin brittle 

 scale and is easily cracked from the eye by a pair of fine pointed 



