MOTOR RESPONSES IN INVERTEBRATES 605 



sity and reached the conclusion presented above. This conclusion rests 

 upon the assumptions that the magnitude of the response to gravity is 

 independent of the axial position of the insect and that there is no differ- 

 ence in the sensitivity of the rhabdomes in different portions of the eye. 

 There is, however, no evidence in support of either of these assumptions. 

 The action of gravity on orientation doubtless varies from maximum 

 when the insect faces downward, to zero when it faces upward, and the 

 effect on deflection of light of a given intensity probably varies from 

 maximum w^hen it is received by the rhabdomes at the posterior edge of 

 the eye, to zero when it is received by those in the center of the anterior 

 portion of the eye. The validity of the conclusion is therefore highly 

 questionable. 



h. Rate of Locomotion with the Two Eyes Equally Illuminated. — Loeb 

 (126, 127) maintains that the light received by the eyes controls the 

 tonus of the muscles in the locomotor appendages of animals including 

 insects, and that their activity is correlated with the muscle tonus in such 

 a way that it varies directly with the intensity of the light in accord with 

 the Bunsen-Roscoe law. But the evidence in support of this contention 

 is far from convincing. 



Mast (140) maintains that in fly larvae with both sides equally illum- 

 inated the rate of locomotion over a distance of 10 cm. is a little higher in 

 strong than it is in weak light. Herms (97) obtained similar results. 

 These observations, as well as those cited above, indicate that the activity 

 in fly larvae varies directly with luminous intensity. But in spite of the 

 quantitative measurements made by all the investigators, it is (because of 

 continuous changes in the intensity of the illumination of the photo- 

 receptors in the larvae, brought about by alternate extension and retrac- 

 tion and swinging from side to side of the anterior end as they crawl) 

 impossible to give anything definite concerning the quantitative relation 

 between the stimulus and the response. 



Dolley (50) concludes that the rate of locomotion in Vanessa with 

 both eyes equally illuminated in a horizontal beam of constant light is 

 practically independent of the intensity of the light, and Minnich (170) 

 reaches the same conclusion in regard to the honey bee. Moore and 

 Cole (173) and Cole (40) conclude, however, that in the Japanese beetle, 

 Papillia japonica and the fruit fly Drosophila melanogaster, "the rate of 

 response of the organism is proportional to the logarithm of the intensity 

 of the stimulus." 



Moore and Cole made their observations as follows: A considerable 

 number of specimens were put into a tall vessel directly under a lamp. 

 After they had aggregated at the top, the vessel was inverted and the 

 time required for half of them to reach the top was measured. Then 

 the vessel was again inverted, etc. The time recorded consequently 

 includes the time required to orient and to get under way as well as the 



