MOTOR RESPONSES IN INVERTEBRATES 601 



In a beam of light they orient with the longitudinal axis at an angle 

 with the direction of the rays and go diagonally across the beam. The 

 angle of deflection is independent of the light intensity; but if the intensity 

 is rapidly increased or decreased after the insects are oriented, they turn 

 sharply from or toward the light, then gradually back until they have 

 assumed their former direction of movement. If the intensity is slowly 

 changed, they do not turn. These responses are therefore typical shock- 

 reactions (Dolley, 49). 



The tendency to turn is much more marked in diffuse light with much 

 reflection from the background than it is in a beam of light with little 

 reflection from the background. It decreases with time and in some 

 insects disappears entirely (Radl, 189; Holmes, 109; Dolley, 49; Minnich, 

 170). Some contend that the decrease is correlated with experience; but 

 Clark (33) found that it is correlated with adaptation to light and that it 

 probably is independent of experience. Mast (149) maintains that if the 

 ventral and the posterior surfaces of the functional eye are covered, there 

 is no tendency to turn. He concludes that the turning is due primarily 

 to stimulation of the rhabdomes in the posterior and the ventral portions 

 of the eye by light reflected from the background. Dolley (49) contends 

 that in vertical illumination the rate of deflection in turning varies 

 directly with intensity, if the source of light is large and the background 

 is white, but indirectly if the source of light is very small and the back- 

 ground is dull black. 



Eristalis on the wing orients fairly accurately. If, after it is oriented 

 in a beam of light, the source of light is raised or lowered, it turns directly 

 upward or downward. When the position of the light is changed, the 

 images in the two eyes are equally and simultaneously changed in loca- 

 tion. The turning is due to this change. It cannot be due to difference 

 in the illumination of the two eyes for they are continuously equally 

 illuminated (Mast, 148). 



If a male firefly on foot or on the wing is so^ne distance from a female 

 firefly when she produces a flash of light, he turns until he faces the direc- 

 tion from which the light came and then proceeds directly toward her. 

 The magnitude of the angle through which he turns consequently varies 

 from zero to practically 180° depending upon the direction in which he 

 faces in relation to the location of the female when the flash of light is 

 produced. This obviously is correlated with the location in the eye of the 

 image of the flash of light. If it is located in the posterior region of the 

 eye, a chain of reflexes is initiated which results in turning through nearly 

 180° followed by direct forward movement. If it is located in the 

 anterior region of the eye, no turning reflexes are initiated. 



If a male firefly glows in the neighborhood of a female, she raises and 

 twists the abdomen until the ventral surface is directed toward him and 

 then glows, no matter where he is located. Thus, after an image of a 



