l8o LIGHT AND THE BEHAVIOR OF ORGANISMS 



and direction of motion of the animal. Other possible com- 

 plications are covered by the unequivocal statement, made 

 and emphasized in this and the following papers on the 

 same subject, that the main feature in all phenomena of 

 heliotropism is the fact that symmetrical points of the 

 photosensitive surface of the animal must be struck by the 

 rays of light at the same angle. It is in full harmony with 

 this fact that if two sources of light of equal intensity and 

 distance act simultaneously upon a heliotropic animal, the 

 animal puts its median plane at right angles to the line 

 connecting the two sources of light. This fact was not 

 only known to me, but had been demonstrated by me on 

 the larvae of flies as early as 1887 in Wiirzburg, and often 

 enough since. These facts seem to have escaped several of 

 my critics." 



It is evident without further discussion that the reac- 

 tions of fly larvae in light from two sources are not in accord 

 with Loeb's conclusions. When exposed in light from two 

 sources of different intensity the stronger does not deter- 

 mine the orientation and direction of motion, nor are sym- 

 metrical points on the photosensitive surface struck by the 

 rays of light at the same angle. 



e. Orientation and movement — (i) perpendicular to the 

 direction of the rays — (2) toward a source of light. — The 

 following experiments bring out clearly the importance of 

 intensity in the orientation of fly larvae. A small horizontal 

 beam of light from a single Nernst glower was thrown on the 

 black paper used in the preceding experiments. In this 

 beam a small vertical post was erected so as to produce a 

 well-defined narrow shadow. By means of a mirror this 

 shadow was illuminated with rays of light either perpendic- 

 ular to or parallel with its edges, as represented in Fig. 29. 

 The intensity of light in the shadow could be regulated by 

 changing the position of the mirror. It was always con- 

 siderably lower than that in the field on either side. 



If a specimen taken from darkness is placed on the plate 

 in the shadow with its anterior end directed toward the 



