INSECTS. 



477 



circulating fluids kept exposed to the vivifying influence 

 of 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 number, and con- 

 sist 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 suffocation. Fig. 

 238 represents the beautiful 



, r . . . , Fig. 238. Magnified portions of the 



mechamsmoiaportion OI the trachea of the Hydrophllus, show- 



trachese of a Silkworm moth, nt spiral tubes and their arran ^ 

 beneath this is a small por- 

 tion of a tracheae, highly magnified, to show the peculiar 

 arrangement of the spiral tubes, which give elasticity and 

 strength to the air-tubes of the Hydrophilus. 



The legs of insects are extremely curious and interesting ; 

 each leg consists of several horny cylinders, connected 

 by joints and ligaments, enclosing within them sets of 

 powerful muscles, whereby their movements are effected. 

 The tarsus, or foot, is generally terminated by two horny 

 hooks or claws, by which the insect holds to the object 

 it is moving upon ; between these hooks, in most species, 

 is situated a cushion, sucker, or two broad flaps (pulwlli), 

 wherewith to take hold of smooth surfaces. In fig. 240, 

 legs are represented, marking peculiarities of structure ; 

 and in fig. 239, a sucker, somewhat resembling the admi- 

 rably-constructed siccker attached to the under-surface of 

 the feet of House-flies, by means of which 



11 They tread the ceiling or inverted floor, 

 And from its precipice depend secure." 



