WALTER E. GARREY 119 



forceps after a day or two. If the right eye had been covered, the 

 Prodacanthus circled to the left, but immediately upon flaking off the 

 covering and flooding the eye with light the direction of the circus 

 motion was reversed and the insect circled to the right, — ^the side 

 from which the black had been removed. The normal eye, owing to 

 its previous exposure to light is fatigued; it has now become physi- 

 ologically darkened although there has been no change in the illumina- 

 tion of that eye. The muscle tonus is affected by the greater sensi- 

 tiveness of the right eye as if it were intensely illuminated. The 

 effect may be produced by the removal of only a very small part of 

 the black covering. The abnormal sensitiveness may persist for 2 

 hours or more but gradually wears off as the eye becomes light- 

 adapted or fatigued, and the robber fly then behaves like a normal 

 one. The experiment is a crucial one, demonstrating that the muscle 

 tonus is proportional to the rate of photochemical reaction in the eye. 

 Effects of Intensity of Light upon Muscle Tonus. — Evidence has 

 already been presented proving that niusclc tonus is maintained by 

 the action of light on the eyes and is lost in the dark, also evidence 

 indicating that the tonus is proportional to the intensity of the illumi- 

 nation of the eyes. This can be demonstrated easily by the changes 

 which result in the circus motions and postures of the insects upon 

 varying the illumination after one eye has been blackened. A 

 butterfly such as Circionis alope thus treated flew in circles with a 

 diameter of 5 or 6 feet in the diffuse light of a room with windows on 

 three sides, while in the sunlight the circles of flight were reduced to a 

 diameter of 2 or 3 feet. Similarly when walking, the diameters of the 

 circles were 2 or 3 feet when in diffuse light, but only a few inches in 

 the sunlight. The degree to which resting butterflies leaned toward 

 the good eye in diffuse light was distinctly increased by reflecting 

 sunlight onto the eye from any direction. The bilateral asymmetry 

 of the legs of Proctacanthus was much more marked in several indi- 

 viduals in the sunlight than when they were shaded, and the circles 

 of movement had a much smaller diameter in the sunlight. Tabanus 

 with one eye blackened, and Eristalis, with the additional blackening 

 of the inner part of the other eye, lean far toward the seeing eye sur- 

 face in sunUght and often simply circle about the tip of the posterior 

 leg as a center, rarely in a circle with a diameter of more than 1 inch. 



