296 THE MECHANICAL FUNCTIONS. 



action and of repose. By means of feet thus 

 armed, the insect can ascend or descend the per- 

 pendicular sides of a rough body with the greatest 

 ease ; but it is scarcely able to advance a single 

 step upon glass, or other polished surfaces, even 

 when horizontal. The hooks at the ends of the 

 anterior pair of feet are directed backwards, those 

 of the middle pair inwards, and of the posterior 

 pair forwards ; thus affording the greatest possible 

 security against displacement. 



Many insects are provided with cushions at the 

 extremity of the feet, evidently for the purpose of 

 breaking the force of falls, and preventing the jar 

 which the frame would otherwise have to sustain. 

 These cushions are variously formed ; sometimes 

 having the appearance of contractile vesicles, de- 

 nuded of hair, and inserted at the base of the claws 

 and between them ;* at other times covered with 

 dense velvety tufts of hair, lining the underside of 

 the tarsi, but leaving the claw uncovered ; and the 

 filaments, by insinuating themselves among the 

 irregularities of the surfaces to which they are 

 applied, produce a considerable degree of adhesion. 

 Cushions are met with chiefly in large insects which 

 suddenly alight on the ground after having leaped 

 from a considerable height : in the smaller species 

 they appear to be unnecessary, because the light- 

 ness of their bodies sufficiently secures them from 

 any danger arising from falls. t 



* These vesicles, which are termed pulvilli, often project con- 

 siderably beyond the claws. The most extraordinary expansion of 

 the membrane covering the underside of the joints of the tarsi occurs 

 in the Cleridce. 



t Several exceptions, however, occur to this general rule: thus 



