RESPIRATION OF 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. 
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 
t 2 
