l68 LIGHT AND THE BEHAVIOR OF ORGANISMS 



tlon. This causes the spiral course to become very irregu- 

 lar and makes it appear as though they were constantly 

 being thrown out of orientation and reorienting. Their 

 general course is toward the source of light but it is a very 

 irregular course. 



d. Mechanics of orientation. — If the direction of the 

 rays of light is changed after the larvae are oriented they 

 all appear to turn directly toward the source of light in its 

 new position without preliminary trial movements. What 

 is the mechanism involved in this apparent direct orienta- 

 tion? If one edge of a cover-glass is supported so that it is 

 a little higher than the other, and if the larvae mounted in 

 water on the slide are forced to travel toward the lower 

 edge, they soon reach a place where the cover- glass is so 

 near the slide that they no longer rotate. Under such con- 

 ditions the animals lie usually on the ventral surface, but 

 some specimens are found on either side or on the dorsal 

 surface. No definite movement is seen in those on either 

 side, excepting occasionally a slight forward motion. But 

 in those on either surface, the anterior end is constantly 

 seen to move from side to side with a slight jerky motion. 

 This lateral movement of the anterior end is undoubtedly 

 due to muscular contraction. If one of the specimens with 

 the dorsal surface up is selected and light thrown upon it 

 from such a direction that the rays strike its side at right 

 angles, the lateral movement toward the side illuminated 

 is at once much increased and the organism turns in that 

 direction. By using two sources of light so situated that 

 the rays cross at right angles in the region where the speci- 

 men is located, and then alternately intercepting the light 

 from each of the two sources, it can be seen clearly that the 

 larva, by muscular movement, turns the anterior end toward 

 the source of light directly. There is no trial reaction in 

 this process. It is an asymmetrical response to an asym- 

 metrical stimulation. This does not however mean that 

 both sides of the organism are stimulated in accord with 

 the theories of orientation of Loeb and Verworn. The reac- 



