64 



INVERTEBRATE PHYSIOLOGY 



as a function of their angular size could not yet be measured exactly be- 

 cause of the time lag of the recording instruments. Motion-picture photo- 

 graphs showed a time duration of less than 80 msec, in jerks of 90°. Thus 

 the duration of jerks smaller than 20° possibly come close to the latency of 

 the optic circuit. If the jerk is executed without any feedback, whether 

 optic or proprioceptive in origin, then the amount of the movement must 

 be based exclusively upon the optic information present before the jerk 

 starts. Consequently the position of the head, after the jerk has finished, 

 should depend not only upon the position of the fly but on the initial po- 

 sition of the head too. Thus, if initial positions are variable, there should be 

 no constant proportionality between the position of the fly (a) and the 

 position of the head (^t) at the end of the jerks, unless the jerks always 



■T3 



a 



Y 



^jf Fly I Prothorax 



Fig. 8. Endpoints of fixation movements of the head plotted against the position of 

 the fly which is faced. Ordinate : deviation of the head (^u.)- Abscissa : deviation of the 

 fly (<r), both from prothorax median plane. -|- : deviation to the right; — : deviation 

 to the left. The experimenter moves the fly in small steps from —25 to -\-2S and return. 

 The procedure is repeated three times. Small circles : head positions reached by con- 

 tinuous movements. Large dots: head positions reached by jerks. Dots are grouped 

 along the lower line, if the fly had reached the position concerned by moving from left 

 to right ; but they are grouped along the upper line, if the fly had moved from right 

 to left. Distance from the fly to the head : 29 mm. 



