RESPIRATION OP INSECTS. 



275 



minute tubes ; and, by the ramifications of these, even the 

 minutest parts of the body are penetrated (fig. 159). These 

 tubes are formed upon a similar plan with the air-vessels 

 of Plants, having a spiral fibre winding inside their outer 

 membranous coat (fig. 158) ; by the elasticity of which 

 fibre, the tube is kept from being closed by pressure. In 

 this manner the air is brought into contact with almost 



Head. 



1st Pair of legs 



1st Segment of\ 

 thorax) 



Origin of wing 

 2d Pair of legs 

 3d Pair of legs 



Tracheae 



Stigmata 



Air- sacs. 



Fig. 159. RESPIRATORY APPARATUS OP INSECT (NEPA). 



every portion of the tissue, and is enabled to act most ener- 

 getically upon it ; and thus the feeble circulation of these 

 animals ( 293) is in a great degree counterbalanced by 



T2 



