INSECTS. 



these are connected by smaller branch pipes, which run across 

 the rings of the abdomen, and distribute an infinite number of 

 smaller tubes to every part of the interior. In insects of great 

 powers of flight, there are likewise reservoirs of air : these are 

 particularly large in the bee. One circumstance connected with 

 the arrangements of the air-tubes specially deserves our admi- 

 ration. It is evident that the sides of canals so slender and deli- 

 cate 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 



FIG. 75. SPIRACLES. OF INSECTS. 



FIG. 76. -AIR-PIPE OF FLY. 



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, aud still 

 preserve the perfect pliancy and softness of their walls. The 

 mode in which this is effected is as follows : Between the two 

 thin layers of which each air-tube consists, an elastic thread is 

 interposed, coiled in close spirals, of sufficient strength and firm- 

 ness to maintain the channel always pervious, but not at all in- 

 terfering with its flexibility ; and this fibre, delicate as it is, may 

 be traced with the microscope even through the utmost ramifica- 

 tions of the air-tubes. Wonderful are the results obtained by the 

 adoption of this new arrangement. Not only is the body of the 

 insect lightened to the uttermost, but the little creature, thus 

 breathing in every part, has its vitality so intensified that it is, 



G 2 



