MECHANORECEPTION 41 



by forces at right angles to its plane or shearing in its plane, as illus- 

 trated by a sheet of rubber stretched over a rectangular frame (Fig. 3 1 ). 

 When the surface is distorted by changing the rectangle to a parallel- 

 ogram there is tension in the direction AC and compression in the 

 direction BD. 



Any force applied to a hollow cylinder sets up shearing forces (see 

 Fig. 31). This is the situation as it occurs in a homogeneous surface to 

 which forces are applied evenly. Insect limbs do not conform to this 

 ideal, but, as Pringle pointed out, the important point is that on a 

 hollow structure such as the insect exoskeleton all stresses can be ex- 



FiG. 31. Diagram to illustrate the resolution of shear forces into com- 

 pression and extension components. A, a flat surface; B and C, a 

 hollow cylinder subjected to twist and bend. Continuous lines, ex- 

 tension. Broken lines, compression. D, model representing actual 

 sensillum; E, model constructed in a circular framework for measure- 

 ment of the effect of compression in different directions. (Redrawn 

 from Pringle, 1938 b.) 



pressed as shearing, which can be resolved into compression and 

 extension components. 



Based upon these ideas, the probable mode of action of a campani- 

 form sensillum can be explained in terms of a second model (Fig. 31). 

 In a sheet of rubber stretched over a frame a circular or oval hole 

 corresponding to the cuticular canal of the sensillum is cut. A domed 

 strip of paper is fastened across the long diameter of the oval aperture. 

 This corresponds to the longitudinal thickening frequently seen in the 

 dome of the sensillum where the nerve fibre is attached. The rest of 

 the cap, assumed to be more elastic, is omitted from the model. The 

 assumption of differential elasticity of the two parts is essential to the 

 theory. 



Distortion of the model from the rectangular shape causes the hole 



