MOTOR RESPONSES IN INVERTEBRATES 603 



On the basis of these facts and others, Mast (147, 148, 149) came to 

 the following conclusions: Orientation in insects is the result of reflexes 

 specifically related to the location of the stimulus in the eyes. Stimula- 

 tion of different regions in either eye induces different series of reflexes in 

 which all the legs are involved. If both eyes are simultaneously stimu- 

 lated, the effect produced in one is modified or neutralized by that 

 produced in the other. If the stimuli in the two eyes are the same in 

 location and magnitude, light has no immediate observable effect. If 

 they are the same in location but different in magnitude, the insect turns 

 toward the stronger. If they are the same in magnitude and different in 

 location, the insect turns toward the one which is farther back.^" After 

 the insect is oriented in a field of light, no matter how complex, orienting 

 stimulation ceases. It is not held on this course by continuous action 

 of the light. It tends to continue in the direction assumed, owing to 

 internal factors. But if for any reason it deviates from the course, it is 

 stimulated and turns back as described above. According to these 

 conclusions the location of the stimuli in the eyes is quite as important in 

 the process of orientation as the relative amount of light received by 

 each of them. 



Clark (35) maintains that the conclusions presented are in accord 

 with all the facts in hand concerning photic orientation in insects except 

 three: (a) increase in stimulating efficiency wdth increase in the size of 

 the source of light; (6) decrease in the angle of deflection in one-eyed 

 insects, with increase in time in light, and (c) decrease in this angle with 

 increase in the intensity of light in low illumination. 



To account for these he adds to the conclusions the follow^ing assump- 

 tions: (a) The " all-or-none " principle obtains in the stimulation of the 

 ommatidia. (6) Variation in the threshold of the ommatidia in any 

 given region is in accord with the normal distribution curve, (c) The 

 threshold of the ommatidia varies directly with light adaptation, {d) 

 The magnitude of the response varies directly with the number of omma- 

 tidia stimulated. 



QUANTITATIVE RELATION BETWEEN STIMULUS AND RESPONSE 



It is well known that the activity of many insects varies with the 

 intensity of the light to which they are exposed. The butterfly Vanessa 

 antiopa, for example, is usually much more active in diffuse sunlight 

 than it is in direct sunlight or in darkness (Parker, 182; DoUey, 49), and 

 blowfly larvae are under certain conditions much more active in strong 

 than in weak light. If, for example, the light intensity is suddenly 

 increased they usually raise the anterior end and swing it violently from 



=*" These statements refer to insects which are photopositive. If they are photo- 

 negative, the opposite obtains. 



