QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 271 



operates to produce orientation, for our experiments on the blow- 

 fly, to test the Bunsen-Roscoe law, clearly show that light of a 

 constant intensity both stimulates and plays an important part 

 in orientation. 



Holmes and McGraw ('13) in experiments on insects have 

 come to the same conclusion in regard to the effect of constant 

 intensity ('13, p. 373) : 



It is not possible we believe to construe phototaxis entirely in terms 

 of differential sensibility. Responses to the shock of transition, whether 

 in the direction of an increase or a decrease of stimulation, may play a 

 part in the orientation of many forms but the continuous stimulating 

 influence of light appears to be in several cases at least the factor of 

 major importance. 



Bancroft's ('13) work, in which he showed not only that there 

 was a distinct reaction to constant intensity present in Euglena 

 but that it was largely the reaction to constant intensity which 

 determined its orientation, shows the untenability of Mast's 

 sweeping statement in one of the forms on which Mast himself 

 Avorked. 



The facts established concerning orientation in the larva of the 

 l)lowfly may be summarized as follows : 



1. When a larva is subjected to a single light, the changes of 

 position, as it swings its head from side to side in the manner char- 

 acteristic of locomotion and orientation, produce changes in the 

 intensity of the light acting on the sensitive anterior end of the 

 animal, due in a large measure to the shadow cast by its own 

 body. 



2. An abrupt change in the intensity of the light acting on the 

 sensitive surfaces produces a reflex toward a "physiologically 

 definite side" — the side on which the muscles are passively 

 stretched. 



3. Repetition of this reflex automatically checks motion toward 

 the light. 



4. In orientation to lights from two sources, the side to side 

 swinging of the head does not produce changes in the effective 

 intensity of the light on the anterior end as a whole. 



