VERMES, FLY LARVAE, AND ECHINODERMS 177 



exposed, retracted and concealed. This is of importance 

 in the orienting reactions as will be seen later (Fig. 31). 



c. Accuracy of orientation. — The orientation of organ- 

 isms is generally supposed to be far more accurate than 

 it really is. This is no doubt due to the fact that many of 

 the observations on light reactions have been made in light 

 which is more or less diffused and the direction of which 

 is not thoroughly under control. In testing the accuracy 

 of orientation in the blowfly larvae they were exposed on a 

 piece of smooth moist black paper on a glass plate in a 

 small horizontal beam of light from a Nernst glower. The 

 course taken by the larvae was traced on the paper in white 

 ink with a small pen held about i cm. in front of the end 

 of the larvae. In this way the movements could be quite 

 accurately traced. Neither the presence of the pen nor 

 the ink on the paper made any appreciable difference in the 

 course taken. The courses of four different individuals are 

 given in Fig. 27. All of the specimens tested deviated con- 

 siderably; those used in Fig. 27, B and C, deviated toward 

 the left in all the trials; the one used in Fig. 27, A, to the 

 right, and that in Fig. 2^], D, to the right in some trials 

 and to the left in others. In Fig. 27, A, the head move- 

 ments are represented more in detail than they are in the 

 others. It will be seen that the lateral movements to the 

 right and the left alternate quite regularly. The posterior 

 end takes a much more regular course than the anterior. 

 In direct sunlight orientation is somewhat more accurate 

 and the lateral movements are not so pronounced. But 

 I failed to find any specimens which " crept with mathe- 

 matical precision in the direction of the rays, . . . exactly 

 parallel to the edge of a shadow," or which " acted as 

 though they were impaled on the ray of light which passed 

 through their median plane," as Loeb states. 



d. Orientation in light from two sources. — In the study 

 of orientation in light from two sources the larvae were 

 exposed on moist black paper just as in the preceding experi- 

 ment, in a field of light composed of two small horizontal 



