52 INVERTEBRATE PHYSIOLOGY 



facts should be noted : First, the stroke has a time duration of about 10 to 

 30 milliseconds. Because of that short interval it should hardly be expected 

 that the stroke is controlled by watching the difference between its direc- 

 tion and that of the prey. Later we shall show evidence that, if present at 

 all, such a control at least is not eft'ective. Second, though the animal tends 

 to bring its head and its prothorax into one line with the prey, it is able to 

 hit a prey which has a considerable lateral deviation from the median plane 

 of the prothorax. 



For these reasons the problem seems to be quite similar to the human 

 case. The direction of the stroke must be determined by a message repre- 

 senting the direction of the prey relative to the prothorax. Consequently, 

 that message should not only contain information about the direction of 

 the prey relative to the head, but of the position of the head too. It may be 

 supposed that the first information is provided by the compound eyes and 

 the second by proprioceptive sense organs which are able to present a 

 message about the position of the head. And indeed mantids possess a well- 

 developed system of neck receptors, and I shall examine these sense organs 

 first. But before going into details, I want to make some general remarks. 



As already indicated by the way I have introduced the problem, it is the 

 functional organization of a system as a whole I want to understand. Con- 

 sequently, though I shall take advantage as far as possible of what is already 

 known about sense organs, efi^ectors, and nerve cells, the aim of this in- 

 vestigation is not — or at least not primarily — to learn more about single 

 elements. In the present case, as we shall see later, we are dealing with four 

 quite distinct functional units. My task will be to discuss how they act to- 

 gether and thus to explain the performance of the whole system by the 

 properties of its parts and their functional interrelations. 



This being clear, I should say a few words about the necessary limita- 

 tions and restrictions. First, I shall deal with the orientation problem only 

 and omit all ethological questions about — for instance — appetitive behavior, 

 drives, and releasing factors. Second, though the learning problem will be 

 touched upon, we shall be concerned with the functioning of the fully de- 

 veloped mechanism in the adult animal only. Third, perfect localization 

 includes an indication of direction and of distance as well. I shall deal with 

 the direction problem only, and only with a special part of it, namely, the 

 orientation in the main horizontal plane. 



First Series 



Now we are ready to start with the first experimental series planned 

 to throw light upon the role played by the proprioceptors. The neck re- 

 ceptors of mantids are of a well-known type. There are two pairs of hair 

 plates each studded with from tens to hundreds of hair sensillae ( Fig. 1 ) . 



