THE PHOTIC KEACTIONS OF SARCOPHAGID FLIES 217 



indirect; and, secondly, locomotion, a movement toward or away 

 from the source of light depending on the existing relation 

 between the organism and the stimulus. 



Many organisms are subject to stimulation on being illuminated, 

 but fewer respond by orientation to the light. Of these (disregard- 

 ing reactions to bright patches of light and intensity alone) the 

 larger number by far move either toward or away from the source. 



The principal concern relates to the two stages inthephototactic 

 process after stimulation and may be expressed by the question: 

 How does the animal orient, and after it is oriented how does it 

 continue on its relatively straight path toward or away from the 

 light? 



Three theories based on reflex re-ponses of the organism have 

 been advanced to explain the method of orientation. The oldest 

 of these theories is the tropism theory of Verworn ('95, pp. 419-446) 

 and Loeb ('97, pp. 439-441), advanced by the latter as an appli- 

 cation of Faraday's conception of lines of force. It is defined by 

 Loeb ('06, p. 140) as follows, ''the animal is turned automatically 

 until symmetrical points of its surface are struck equally by the 

 lines of force. As soon as this occurs the animals must keep this 

 orientation, and therefore have no further choice in the direction 

 of their motions." 



The second theory is that of Jennings (the method by trial error) 

 which is not entirely new, but in its present application is dis- 

 tinctly in advance of previous views. The following extracts from 

 the writings of Jennings ('04, p. 237) will serve to define the theory. 

 ''On receiving a stimulus that induces a motor reaction, they try 

 going ahead in various directions. When the direction followed 

 leads to a new stimulus, they try another, till one is found which 

 does not lead to effective stimulation" (p. 252). "This method 

 involves many of the fundamental qualities which we find in the 

 behavior of higher animals, yet with the simplest possible basis in 

 ways of action : a great portion of the behavior consisting often of 

 but one or two definite movements, movements that are stereo- 

 typed when considered by themselves, but not stereotyped in their 

 relation to the environment. This method leads upward, offering 



