120 HELIOTROPIC MECHANISM 



On the other hand, if these ihes are shaded the diameter of the circles 

 increases to 3 or 4 inches, while in the dim light of our dark room 

 with open door, no tendency to circle was noticeable. 



These effects are a function of the intensity of the illumination and 

 are independent of all orienting effect of the light from a single source. 

 Experiments devised to demonstrate this fact were performed by the 

 author in 1912 and were described in lectures at Woods Hole that 

 year." The experimental insects were placed on the bottom of an 

 illumination chamber made from a cylindrical dish, 20 inches in diam- 

 eter and 12 inches deep, lined throughout with white blotting paper; 

 the dish was covered with ground glass, and illuminated centrally 

 from above by tungsten lights. Peep holes were provided for observa- 

 tion. With a 10 watt light, although one eye had been blackened, 

 the insects moved about indifferently or at least had no difficulty 

 leaving the floor and ascending the walls of the dish. W^ith a 25 

 watt light they kept on the bottom; when a 100 watt light was used 

 the circles were never over 6 or 8 inches in diameter, while a light of 

 400 watts caused a pivoting in one spot. 



Results which lead to the same conclusions were obtained by par- 

 tially excluding hght from one eye by painting with collodion in ether- 

 alcohol solution or with shellac in alcohol. The effects of a single 

 layer of collodion were hardly noticeable. Successive layers, espe- 

 cially when milky, produced progressive narrowing of the circles of 

 movement. Shellac had the same progressive effects until the result 

 of total blinding was produced. 



The results of these experiments justify our assertion that the 

 muscle tonus varies directly with the intensity of the illumination, 

 and that the asymmetry of the muscular tone, upon which circus 

 motions depend, is determined by the difference in illumination of 

 the two eyes. The same conclusions are reached by summing the 

 effects of gravity with light, as described below. 



Experiments with Heliotropic Insects Placed upon a Vertical Surface. 

 — Butterflies, with normal eyes, and common flies walk directly up a 



^^ Similar experiments have been described b}' Holmes and McGraw (/. Animal 

 Behavior, 1913, iii, 367) who describe "orienting effects with a constant illumina- 

 tion." Dr. Minnich under the direction of Professor G. H. Parker has made 

 quantitative determinations of circus motions under similar experimental condi- 

 tions; we regret that they are not yet available for reference. 



