110 



INSECTS. 



bee. One circumstance connected with the arrange- 

 ment of these air-tubes specially deserves our admi- 

 ration. It is evident that the sides of canals so 



Fig. 75. — SPIRACLES of insects. 



slender and delicate would inevitably collapse and 

 fall together, so as to obstruct the passage of the air 

 they are destined to convey, were not some plan 

 adopted to obviate such an occurrence ; and the only 

 mode of providing against this would appear to be 

 to make their walls stiff and inflexible. Inflexibility 

 and stiffness would, however, never do in this case, 

 where the tubes in question have to be distributed in 

 countless ramifications through so many soft and 

 distensible organs, and the problem, therefore, is how 

 to maintain them permanently open in spite of 

 external pressure, and still preserve the perfect pliancy 



i 



Fig. 76.— aiu-pipe of fly. 



and softness of their walls. The mode in which this 

 is effected is as follows: — Between the two thin 



