MOTOR RESPONSES IN INVERTEBRATES 607 



with a degree of precision worth while it is necessary rigidly to control 

 adaptation and the localization of the stimulus in the eye. 



Hartline (78) did this in observations on the action current in the 

 bumble bee Bombus and the grasshopper Melanoplus. He found that 

 the initial action current (millivolts) in dark-adapted eyes is directly 

 proportional to the time of exposure if the intensity is constant, and to the 

 logarithm of the intensity if the time of exposure is constant, but that 

 this obtains only over limited ranges of intensity. This method yields 

 very consistent and valuable results, but it is obvious that these results 

 do not directly express the quantitative relation between the stimulus 

 and the motor response. 



THE NATURE OF THE STIMULUS 



It is very generally held that light causes, directly or indirectly, 

 chemical changes in the eyes of insects and that these changes result 

 in the production of impulses which pass to the muscles and induce the 

 responses. 



Loeb (127), Garrey (75), et al. contend that the light acts continu- 

 ously, that the chemical changes in the eyes are directly proportional to 

 the amount of light received in accord with the Bunsen-Roscoe law, and 

 that the magnitude of the response is directly proportional to the photo- 

 chemical changes, as in Hammond's mechanical dog, which is motivated 

 and directed by the action of light in selenium cells symmetrically 

 situated on opposite sides. 



If these contentions are valid, intermittent light should have the 

 same stimulating efficiency as continuous light. It has been demon- 

 strated that if the frequency of intermittent light is high this obtains in 

 blowfly larvae (Patten, 183), in the tachina fly (Archytas), the butterfly 

 (Vanessa), and the drone fly (Eristalis) (DoUey, 50, 51, 52), in dragon-fly 

 larvae (Sjilzle, 192), and in the honey bee (Wolf, 229). 



This is, however, to be expected, for just as it is impossible to dis- 

 tinguish between a straight line and the arc of a circle, if the circle is 

 sufficiently large, it is impossible to distinguish between continuous and 

 intermittent light, if the frequency is sufficiently high. The relation in 

 question is, therefore, of importance only when it is specifically correlated 

 with the frequency (Mast, 148). 



Mast and Dolley (157), in observations on Eristalis, ascertained the 

 critical frequency for different intensities, that is, the lowest frequency 

 in intermittent light, which is for this insect indistinguishable from 

 continuous light of the same intensity. It was found that this frequency 

 varies directly with the intensity. Salzle (192) and Wolf (227 to 229) in 

 observations on dragon-fly larvae and the honey bee, ascertained this 

 frequency over a much wider range of intensities and came to the same 



