MECHANORECEPTION 43 



Pringle constructed another model in which there were two strips of 

 paper, one on each surface (Fig. 31). The movement measured 

 between the two and the relation between the angle of the applied 

 compression force and the resistance of the detecting electrolytic cell 

 is shown in Fig. 32. 



If it be assumed excitation occurs only with increased doming, then 

 the neuron will be excited when the compression component of the 

 shear force makes an angle of less than a certain critical value with the 









^^f ..fl^ 





/i) 



<tBB^ 







Fig. 33. Details of the orientation of the groups of campaniform sensilla on 

 the third leg oiPeriplaneta. (Redrawn from Pringle, 1938 b.) 



direction of thickening of the cap membrane. The sensitivity of the 

 sensillum will depend, among other things, on the absolute length of 

 the long diameter of the cap. Sensilla with parallel orientation will 

 respond to the same type of shear force. Each group will act as a unit 

 and, if innervated from the same fibre, will extend the range of these 

 fibres quantitatively but not quaHtatively (Fig. 33). This condition 

 occurs in the sensilla of the palps (Pringle, 1957). 



Although there is yet no direct evidence that the sensillum has 

 undirectional sensitivity, considerable circumstantial evidence sup- 

 ports the idea. In many instances groups of sensilla are orientated in 

 the same direction. The sensilla on the tarsus have their long diameters 



