266 



INSECTA. 



viscera and internal organs. The muscular system, the legs, the 

 wings, the alimentary canal, and even the brain itself, are permeated 

 in all directions by these air- conducting tubes, and thus the 

 oxygen penetrates to every corner of the body. 



(307.) There is one Fig. lie. 



circumstance connected 

 with the tracheae, which 

 is specially deserving 

 of admiration, whether 

 we consider the obvious 

 design of the contriv- 

 ance, or the remarkable 

 beauty of the struc- 

 ture employed. It is 

 evident that the sides 

 of canals, so slender and 

 delicate as the tracheae 

 of insects, would in- 

 evitably collapse and 

 fall together, so as 

 to obstruct the passage 

 of the air they are 

 destined to convey ; 

 and the only plan 

 which would seem cal- 

 culated to obviate this 

 would appear to be, to 

 make their walls stiff 

 and inflexible. In- 

 flexibility and stiffness, 

 however, would never 

 do in this case, where 

 the vessels in question 

 have to be distributed 

 in countless ramifica- 

 tions through so many --:->, 

 soft and distensible viscera ; and the problem, therefore, is, how 

 to maintain them permanently open, in spite of external pres- 

 sure, and still preserve the perfect pliancy and softness of their 

 walls. The mode in which this is effected is as follows : Be- 

 tween the two thin layers of which each air-vessel consists, an 



