330 THE MECHANICAL FUNCTIONS. 



Tevolved in a spherical socket. The principle of 

 this compound motion is the same as that em- 

 ployed on ship-board for the mariner's compass, 

 and other instruments which require to be kept 

 steady during the motion of the ship. For this 

 purpose what are called gimbals are used, the 

 parts of which have two axes of rotation, at right 

 angles to each other, so as to enable the compass 

 to take its proper horizontal position, indepen- 

 dently of any inclination of the ship. 



The tibia, or shank (s), is joined at an acute 

 angle with the femur ; and is frequently either 

 beset with spines, or else notched or serrated. 



The tarsus, or foot (u), is the last division of 

 the limb : it is divided into several joints, 

 which have been supposed to represent those 

 of the toes of quadrupeds. The joints are 

 generally of the hinge kind, but some are met 

 with of a more rounded form, and approaching 

 to that of the ball and socket. The whole 

 structure is most admirably adapted to its exact 

 application over all the inequalities of the sur- 

 faces on which the insect treads. But as the 

 habits and modes of life of this numerous class 

 are exceedingly diversified, so the form of the 

 feet admits of greater variety than that of any 

 other part of the limb. 



The feet of insects diverge, and spread over a 

 wide surface ; thus extending the base of sup- 

 port so as to ensure the stability of their bodies 



