68 INVERTEBRATE PHYSIOLOGY 



Discussion 



It has been demonstrated in the second series of experiments that, at 

 the time before the stroke is released, the prey is faced by a continuous 

 optic feedback process. At equiUbrium, the fixation hne does not center 

 the prey but deviates from it by an amount proportional to the angles be- 

 tween the prey and the median plane of the prothorax. So far the assump- 

 tions made by our hypothesis have been confirmed. All further conclu- 

 sions from this series, and their interpretation in relation to the problem, 

 depend upon whether it can be ascertained that the saccadic head move- 

 ments are controlled in essentially the same way as the continuous ones 

 or not. 



The first two results (1,2) of the last experiment obviously corroborate 

 the assumption made before, that the saccadic movements too are con- 

 trolled by feedback processes. The result not only shows that there is an 

 efifect of the proprioceptors on head movement, but it shows just that sort 

 of influence which the theory predicts in the case of two counteracting 

 control circuits. For, if the proprioceptive circuit be cut ofif, the optic circuit 

 should be more eft'ective in minimizing the deviation of the prey from the 

 fixation line (f>, and hence the ratio /x/o- (since <^ = o- — /i) should be larger 

 after operation than before. 



Yet, if we compare the larger spread of the head positions reached by 

 saccadic movements with the negligible fluctuations of those reached by 

 continuous ones (cf. Figs. 9 and 10), it is fairly clear that the endpoints 

 of the jerks are not identical with the values which correspond to the final 

 steady state of the feedback process. Consequently, at least one restriction 

 must be made, namely, that in the jerks the feedback is blocked before the 

 equilibrium is reached. As an alternative hypothesis, it could be assumed 

 that a jerk is a simple reflex initiated by the deviation of the fly (<^), and 

 not at all controlled by the effects of its output. But then, as already men- 

 tioned in the last section, the linear regression functions of /^ on o- obtained 

 in all these experiments would be difficult to explain. Thus, the data avail- 

 able at the present state of the analysis are best fitted by the assumption 

 that both sorts of head movements are controlled by optic and propriocep- 

 tive feedback, though the dynamic qualities of the two systems may be 

 fundamentally dififerent.^ 



2 Future analysis may reveal that the continuous process operates near the theoret- 

 ical optimum of velocity and dynamic stability, so that any further enlargement of 

 speed (in the jerks) would cause overshooting and oscillation, were the circuit not 

 interrupted from time to time. Such a dynamic dichotomy in the ways of fixation 

 seems to be plausible from a biological point of view ; the pursuit of a moving prey 

 which is far beyond the reach of the stroke demands quickness but no precision. The 

 reverse is true if, after a long stalking, the direction of the stroke has to be de- 

 termined. 



