PREY CAPTURE IN MANTIDS 



63 



Fig. 7. Device for measuring the fixation-deficit. The animal 

 is fixed at the prothorax, the head being free for rotation about 

 the vertical axis. The deviation of the head and of the prey from 

 the median plane of the prothorax are measured on the same 

 dial. 



only three animals being investigated in the first and another three in the 

 second device. But because the result seems to be fairly interesting with 

 regard to the problem as well as to the concept adopted, I feel I should not 

 withhold it here. 



It turned out that two quite different sorts of head movements occur in 

 mantids, similar to the relevant eye movements in man. There are quick 

 saccadic head movements, on the one hand, and smooth continuous move- 

 ments, on the other. The first are observed if the prey appears and moves 

 beyond a distance of about 30 mm. — that is, in Parastagmatoptera, nearly 

 double the reach of the stroke. About 30 mm. distance both types are seen ; 

 if the prey comes nearer, it is followed by continuous movements only. 

 One may be certain that, in the continuous movements, the optic control 

 circuit is working in fact, but it must be doubted whether that be true 

 within the short intervals of the head jerks. The time duration of the jerks 



