PREY CAPTURE IN MANTIDS 69 



With these restrictions born in mind, I want to discuss some of the 

 further consequences of the second experimental series. The result last 

 presented (4) indicates that the endpoints of the jerks after deafferenta- 

 tion are distributed at random around the true points of fixation (the left 

 or right side of the fly). If the same occurs in the continuous movements, 

 so that the fixation-deficit after deafiferentation is zero, some interesting 

 conclusions can be drawn on the qualities of the system. For then either the 

 optic or the neck motor unit must be an "integral action controller," that 

 is, the output of the system depends on the time integral of its input, and 

 hence only comes to rest if the input is zero. Evidently the neck motor tmit 

 operates this way and the optic unit does not ; otherwise there would be 

 no fixation-deficit in the normal mantis. Consequently, since the input of 

 the neck motor unit is controlled by the difference between the optic and 

 the proprioceptive center messages, both must be equal at equilibrium. 

 And this again means that all disturbing forces acting upon the neck motor 

 system are completely ruled out within the limitations set by the capacity 

 of the muscles. It should be noted that this is supported by the result of 

 the loading experiment (ci. first series). Thus, even if future research 

 should show that the fixation-deficit after deafferentation slightly differs 

 from zero, the following statements can be considered as good approxima- 

 tions, namely : 



Ac (total) — ^'°P" 



•^(prop) 



and hence the function of the deviation of the stroke (k) on the deviation 

 of the prey (a) at equilibrium is 



A (Stroke) ' (l~hF) 



' + ' 



•^ (opt) -^ (prop) 



where F is the factor which determines the additional influence of the 

 proprioceptive center message on the direction of the stroke (cf. 

 Fig. 6c). 



Finally, the efficiency of the functional organization as revealed by the 

 present analysis may be briefly considered from a biological point of view. 

 Four main advantages are combined by the system : 



( 1 ) The head can be moved and hence the prey can persistently be 

 faced by the region of the compound eye with the best visual acuity. 



(2) Nevertheless the prey can be localized correctly, even if it deviates 

 from the body axis. 



(3) The hitting performance is independent of external and internal 

 influences which act upon the neck muscles. 



