2l6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



positive terrestrial amphipods and several different flies 

 with one eye blackened turn continuously toward the func- 

 tional eye. Parker (1903, p. 463) working on Vanessa 

 antiopa, and Radl (1903, p. 61) on the flies Dexia carini- 

 frons and Musca domestica, obtained similar results. The 

 fact that these organisms, all of which are positive, thus turn 

 continuously toward the functional eye seems to show that 

 the orienting stimulus is not necessarily and exclusively due 

 to a decrease of intensity in these forms, as it is in positive 

 Euglena and many other organisms. It may be due to the 

 continued action of light on the eye. A change of light 

 intensity does however undoubtedly produce a stimulus 

 which may result in orientation. 



The performance of circus movements has frequently 

 been brought forward in support of Loeb's theory of orien- 

 tation stated in the following quotation (1906, p. 139): 

 " It seems that in animals the region at the oral pole is, as 

 a rule, more sensitive than the rest of the body. Conse- 

 quently the tension of the muscles determining the position 

 of the head or oral pole is more intensely affected by dif- 

 ferences in the intensity of light than that of the muscles 

 of the rest of the body. The head is consequently bent 

 until its symmetrical photosensitive points are again struck 

 at the same angle by the rays of light. The tension of the 

 symmetrical muscles of the head then again becomes equal, 

 and the head must remain in this position unless other forces 

 disturb its orientation. The rest of the body follows the 

 orientation of the head." 



The precision with which some organisms with but one 

 functional eye perform circus movements does indeed 

 appear to add support to this explanation of orientation. 

 Recent investigations have however thrown considerable 

 doubt on the earlier interpretation of these movements. 

 It has been found that they are not so regular and constant 

 as was formerly supposed. Carpenter (1908, p. 486), experi- 

 menting on Drosophila with one eye blackened, observed 

 that they " crept in a fairly direct path toward the light, 



