70 INVERTEBRATE PHYSIOLOGY 



(4) The "calibration" of the mechanism which determines the direction 

 of the stroke can be based solely upon the amplification factors of the pro- 

 prioceptive and optic units. Thus it is at least not inconceivable that the 

 correct adjustment is predetermined by the structure of the sense organs 

 and the nerve connections involved. 



Summary 



The sensory-motor coordination which enables mantids to hit their 

 prey is analyzed by recording the hitting performance under controlled 

 experimental conditions. It is found that : 



(1) Normal mantids (Parastagmatoptera unipunctata) hit about 85% 

 of the flies they intend to capture. If the proprioceptors of the neck region 

 are eliminated by nerve section, the hitting performance is irreversibly 

 reduced to 20-30%. 



(2) If the head is rigidly fixed on the prothorax in the median position, 

 the performance is normal; but it decreases to 25% if the head deviates 

 from the body axis by 10-30°. The prey is missed to the left if the head 

 has been turned to the right, and vice versa. 



(3) If head fastening and unilateral elimination of the proprioceptors 

 are combined, the effects of both are superposed. The loss of one-half of 

 the neck receptors is equivalent to an angular deviation of the head less 

 than 20°. 



(4) If the (free) head is loaded by an extraneous force, the achieve- 

 ment remains normal until the load surmounts twice the head weight at 

 twice the head diameter. 



It is concluded that the direction of the stroke depends upon feedback 

 processes which control the position of the head as follows : The fixation 

 movements of the head, which precede the release of the stroke, are steered 

 by the difference between the optic-center message (which is a function 

 of the angle between the prey and the fixation-line) and the proprioceptive- 

 center message (which is a function of the angle between the head and the 

 body axis ) . If the fixation movements have come to rest, the direction of 

 the stroke is determined by the optic and (to a smaller extent) the pro- 

 prioceptive-center messages, which then both contain the required infor- 

 mation. 



The hypothesis is cross-checked by measuring the position of the head 

 at the end of the fixation movements. It turns out that the fixation line 

 does not center the prey, but deviates from it by an amount proportional 

 to the angle between the prey and the body axis. As predicted by the theory, 

 this deviation is diminished after total proprioceptii^e deafferentation. 



