234 THE MECHANICAL FUNCTIONS. 



to each other, so as to enable the compass to talie its proper 

 horizontal position, independently 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 (r,) 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 surfaces 

 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 sur- 

 face; thus extending the base of support so as to ensure the 

 stability of their bodies in the most perfect manner. When 

 the legs are very long, as in the Tipula,^^ the body seems, 

 indeed, more to be suspended than supported by them; con- 

 trary to what obtains in quadrupeds, where the feet are 

 more immediately underneath the points at which they are 

 connected with the trunk. 



The last joint of the tarsus is generally terminated by a 

 claw, which is sometimes single and sometim.es double, and 

 which contributes to fasten the foot, under a variety of cir- 

 cumstances, both of action and of repose. With feet thus 

 armed, the insect can ascend or descend the perpendicular 

 sides of a rough body with the greatest ease; but it is scarce- 

 ly 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 

 afibrding the greatest possible securit}^ against displacement. 



• It has been conjectured that the object In furnishing- this insect with leg's 

 of so great a length is that of enabling it to walk among- blades of g-rass. 



