88 THE ANIMAL KINGDOM. 



delicate spiral vessels, called tracheae, or air-tubes, which 

 ramify into the minutest organs. As you observe these 

 spiracles, look also at these slides, which are very commonly : 

 found in all collections. 



TRACHEA OF DYTISCUS. - 



You see they greatly resemble the spiral vessels of plants; 

 within the outer membrane an elastic fibre winds round and 

 round in close and regular coils, and then comes another 

 thin transparent membrane, closing it in and securing it 

 from disturbance. When these tubes- are pressed flat and 

 are large, as the tracheae of Dytiscus, then the double wall 

 of fibre crossing each other gives an appearance of watered 

 silk. Vessels such as these in the tongues, wings, -and 

 throughout the body, are easily dissected for examination, 

 by opening the abdomen and floating off the fine white 

 threads which branch off on either side from every spiracle. 

 This brings us back to the spiracle itself, which is on every 

 segment of the abdomen, an oval opening defended by those 

 beautiful arborescent hairs, preventing dust or particles of 

 harmful substance from entering in and lacerating the deli- 

 cate tracheal vessels. 



SPIRACLES OF COCKCHAFER.- 

 (Melolontha.) 



Here is a difference of structure in the spiracles of the 

 larva of Cockchafer, which burrows in the earth to a great 

 depth, and whose naked body has no defence for its tracheal 

 aperture ; therefore the spiracle, though very small, is doubly 

 protected by a framework of bars, stretched from side to 

 side of the thickened margin, and a membrane dotted with 

 minute holes covers these again, effectually protecting the 

 trachese, whilst it freely admits the air. 



SPIRACLE OF FLY, 



is a modification of the spiracle of Dytiscus. It is inter- 

 laced with branching fibre. 



