58 HUMBLE CREATURES. 



beautifully constructed organs, the air passes into 

 fine tubes called trachea, whence it is conducted iato 

 other receptacles varying in form and size. Two of 

 these are capacious sacs, occupying a considerable 

 portion of the abdomen (PL III.) ; whilst one main 

 tracheal or tubular vessel extends along each side of 

 the abdomen and thorax; and from these, minute 

 branches ramify in such a manner as to convey the 

 needful supply of air to every part of the insect's 

 body. Let us first examine the spiracles or stigmata, 

 that admit the air into the body. If these had been 

 simple incisions or apertures ia the outer integument, 

 they would have admitted along with the atmosphere 

 minute particles of dust, and the tracheal tubes would 

 soon become so choked up as to prevent the free 

 circulation of the air. To obviate this difficulty, 

 therefore, nature has protected the entrance by a 

 species of sieve or screen, which is as beautiftd as it 

 is effective, and is formed in the following manner : — 

 From every part of the circumference of the aperture 

 there proceed towards the centre a great number of 

 short arborescent tnmks (PI. VII. fig. 3), whose ianu- 

 merable minute ramifications become so interlaced 

 as to form a complete network across the entrance, 

 and beyond which every particle of foreign matter is 

 effectually prevented from passing. But when the 

 air has entered the tracheae or circulating tubes, 

 another difficulty presents itself; for these tubes are 

 necessarily very compressible, and the rapid move- 

 ments of the insect must often cause them to collapse 



