^^ INVERTEBRATE PHYSIOLOGY 



in head position which cannot be corrected or, as can be said as well to 

 deviations from the intended head position. Removal of the hair plates 

 causes a sharp decrease in achievement, but the effect is eliminated or even 

 reversed simply by fastening the head in a suitable position ; thus it must 

 be assumed that these organs are involved in a system which controls the 

 head movement-in a system which provides for the intended head position 

 to be reached in fact. 



To give a precise form to this hypothesis, so that conclusions can be 

 derived which can be tested further by experiment, I shall" take advantage 

 of methods and theories recently developed to explain the functioning of 

 systems like that with which we are dealing. I mean the modern theory of 

 automatic control and of control systems in general developed during the 

 last decades to a high level of precision and universality. I shall make an 

 attempt to apply that concept to the system which in mantids controls the 

 movements of the head. 



If a prey comes into sight, the mantid turns its head to face it Thus 

 there must be a functional unit which transforms the position of the prey 

 relative to the compound eyes into a central nervous representation of that 

 position. The latter must be transmitted to the centers controlling the neck 

 muscles, which then start a head movement determined bv that message 

 with regard to direction, amount and/or speed. But as soon as the head 

 changes itsposition, the optic message is changed too, so that the output 

 of he physiological pathway must necessarily influence its own input To 

 make that completely clear, I shall plot it diagrammaticallv (Fig 6a)' 



1 shall define all directions or positions by the angular deviations from 

 the median plane of the organ concerned, the axis of reference for all angles 

 bemg the vertical axis of the head movement. We have to distinguish ( 1 ) 

 the angle between the prey and the head, that is the optic input ; (2) the 

 angle between the head and the prothorax ^, that is the neck motor output 

 and the proprioceptive input as well ; and (3) the angle between the prey 

 and the prothorax a. Then we should consider the optic control unit con- 

 verting the angle <^ into a central nervous message <^„ and the neck motor 

 unit which transforms <f>, into a head deviation ,,. Since 



— (T — jj.. 



Thus, as indicated by the arrows, information is transmitted in one direc- 

 tion only, and flows within a closed loop, the output being negatively fed 

 back to the input. » t> j 



Now I shall take into account the final steady state of the system only- 

 that IS, the position after all movements have come to rest, all actions and 

 forces being in complete equilibrium. If, for the sake of an easier introduc- 



