INSECTS' FEET. 



587 



kept exposed to the vivifying influence cf the atmosphere, 



the necessity for more complicated and cumbersome 



breathing organs is at once obviated ; and the whole body 



is at the same time rendered lighter. The spiracles are 



usually 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 



tf ,. v,. c\n i 



SUnOCatlon. lg. JO 4: re- 



presents the beautiful me- 

 chanism of a portion of the tracheae 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, <kc. " 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 



S- 264. Magnified portions of the 

 trachea of the ffydrophilus, show- 

 ing spiral tubes and, their arrange- 



ment - 



