110 HELIOTROPIC MECHANISM 



extreme tilting of the body toward the side of the uncovered eye sur- 

 face. All the legs on this side are flexed until hidden under the body 

 and the anterior leg is adducted far toward the side on which the black 

 has been applied and may even be visible beyond the body on that 

 side. In preening the abdominal segments with the posterior legs, 

 the leg on the blackened side rubs only the lateral and dorsal aspects 

 while on the side of the functioning eye the preening is confined to the 

 lateral and ventral aspects of the abdomen. 



The fruit fly, Drosophila, owing to its precise heliotropic reactions 

 would be material of choice for these studies. When one eye is 

 brilliantly illuminated, these flies show the typical flexion of the legs 

 on that side. The blackening of the eye is a time-consuming pro- 

 cedure which offers practical difficulties due to the diminutive size; 

 one cannot be certain that injury has not been inflicted, nor that com- 

 plete blackening is accurately accomplished. Nature has performed 

 the experiment for us however. Among the types of Drosophilce 

 raised by Professor T. H. Morgan, is a type possessing one normal red 

 eye, the other eye being white, and blind or defective. Professor 

 Morgan called my attention to the fact that these flies move in circles 

 toward the normal eye. He gave me the opportunity, in one instance, 

 of convincing myself that light acting on these one-eyed forms produces 

 the same effects as on normal flies with one eye blackened. More 

 recently I have studied many of these flies and found that the typical 

 asymmetry in the position of the legs is characteristic, the legs on the 

 side of the normal eye being flexed and even concealed beneath the 

 body, while those on the side of the defective eye )vere extended. 

 The flies moved in circles from the time they were hatched until they 

 died; they never learned to correct the abnormal physiological con- 

 dition of the musculature. 



after blackening one eye the typical results were obtained, in exaggerated form, 

 by blackening the inner half of the other eye. The writer has been led to the 

 belief that in most of these forms each eye can function like both eyes of other 

 forms. Each eye controls the muscle tonus of symmetrical groups of muscles on 

 both sides of the body. Physiologically speaking these forms are bilaterally 

 cyclopic. This possibility should be borne in mind when working with forms in 

 which it is found that blackening of one eye does not produce the expected results; 

 thereby one of the stumbling blocks to agreement on the heliotropic mechanism 

 will be removed. 



