INSECTS FEET. 



587 



kept exposed to the vivifying influence of the atmosphere, 

 the necessity for more complicated and cumbersome 

 breathing oi'gans is at once obviated ; and the whole body 

 is at the same time rendered lighter. The spiracles are 

 usuallj'" nine or ten in num- 

 ber, and consist of a horny- 

 ring, of an oval form. The 

 air-tubes are exquisitely- 

 composed of two thin 

 membranes, between which 

 a delicate elastic thread or 

 spiral fibre, is interposed, 

 forming a cylindrical pipe, 

 and keeping the tube always 

 in a distended condition ; 

 thus wonderfully preserving 

 the sides from collapse or 

 pressure in their passage 

 through the air, which 

 otherwise might occasion ^J?- 264 



suffocation. Fig. 264 re- 



Magnified portions of the 

 trachea of the Hydrophilus, show- 

 ing spiral tubes and their arrange- 

 ment. 



presents the beautiful me- 

 chanism of a portion of the trachese of Hydropilus, 

 showing the peculiar arrangement of the spiral tubes, 

 which give it elasticity and strength. 



The legs of insects are extremely curious and interest- 

 ing ; each leg consists of- several horny cylinders, connected 

 by joints and ligaments, inclosing within them sets of 

 powerful muscles, whereby their movements are effected. 



Feet of Flies, d'c. — " The tarsus, or foot of the Fly, con- 

 sists of a deeply bifid, membranous structure, pulvillus ; 

 anterior to the attachment of this part to the fifth tarsal 

 joint, or the upper surface, are seated two claws, or ' tarsal 

 ungues,' which are freely movable in every direction. 

 These ungues differ greatly in their outline, size, and rela- 

 tive development to the tarsi, and to the bodies of the 

 insects possessing them, and in their covering \ most are 

 naked over their entire surface, having however a hex- 

 agonal network at their bases, which indicates a rudimen- 

 tary condition of minute scale-like hairs, such as are common 

 on some part of the integument of all insects. Flexor and 



