216 WILLIAM BRODBECK HERMS 



Clearly, this element must have a profound influence on the 

 behavior of the individuals. Certainly, the seeming indifference 

 to light on the part of the individual whose trail is reproduced 

 in fig. 15, is nothing more than the result of the after-effects of 

 previous stimultion, since the trail soon conforms to the new 

 direction of the light rays. Using this individual as an example, 

 it is to be noted that the larva was perfectly negatively photo- 

 tactic, moving away from the light in the direction of the arrow. 

 The directive light was turned off when the larva was at the point 

 X, and thence to the point Y the larva continued on its way in 

 total darkness undirected save for the after-effects; at Y the larva 

 was suddenly illuminated by directive light from in front. Evi- 

 dently regardless of the new stimulation it continued on its way 

 (except for an increase of random movements) directly toward the 

 light. At this juncture there is an apparent response, which, 

 unless the history of the case were known, might be interpreted as 

 positive phototaxis. The real conditions are obscured by the 

 after-effects. 



Though the experimental evidence is not sufficiently complete 

 to warrant a general statement, it appears from the observations 

 made, that the after-effects are (within certain bounds) propor- 

 tional to the intensity which produced them. 



This phenomenon is accordingly one which has an obscuring 

 effect, comparable to that of mechanical stimulation referred to 

 on p. 191, and noted in other organisms by several investigators, 

 among them Towle ('00) for Cypridopsis, Holmes ('056, p. 319) 

 for Ranatra, and Walter ('07, p. 130) for planarians. 



B. How does the animal orient to light? The sarcophagid fly- 

 larvae orient negatively to light and move away from the source, 

 following very precisely the path of the rays. This behavior is 

 also adhered to even when the course lies in a field of light increas- 

 ing in intensity in the direction taken by the creeping larva, result- 

 ing, however, in uncertainty of orientation and a consequent 

 irregular path. 



Two distinct stages may be recognized in the process of photo- 

 taxis after stimulation; first, orientation, which may be direct or 



