CHAPTER XII 



REACTIONS TO LIGHT WHICH DO NOT RESULT IN 

 AGGREGATION OR ORIENTATION 



We have demonstrated in the preceding pages that reac- 

 tions to sudden changes of Ught intensity may result in 

 orientation followed by aggregation, or in aggregation with- 

 out orientation. In either case the immediate response 

 to the change of intensity is an abrupt change in direction 

 of motion called Schreckbewegung, or avoiding reaction. 

 Most of the organisms considered respond thus to an 

 increase of intensity under some conditions and to a 

 decrease under others. The immediate response under the 

 two conditions is precisely the same; but in case of orien- 

 tation the former leads to locomotion away from the source 

 of light, the latter to locomotion toward it; and in case of 

 aggregation, the former results in collections in regions of 

 relatively low light intensity, the latter in regions of rela- 

 tively high intensity. 



There are many organisms which respond to sudden 

 changes of light intensity much like those referred to above. 

 They contract suddenly or change their direction of locomo- 

 tion abruptly, but these reactions ordinarily result neither 

 in orientation nor in aggregation. Most of these reac- 

 tions are responses to shadows, i.e. to a sudden decrease 

 in light intensity. These organisms do not all however 

 react in the same way, and may consequently be divided 

 into several groups as follows : — i . Reactions to shad- 

 ows — protective; 2. Reactions to shadows — procuring 

 food; 3. Reactions to sudden increase of light intensity; 

 4. Reactions to light caused by the effect of continued 

 illumination. 



246 



