AND PHYSIOLOGY. 



277 



of some insects, and the tracheae are very minute, ramifying 

 through the most delicate organs of the body, which plan 

 allows a rapid aeration of the blood, and greatly assists in 

 diminishing the specific gravity of the animal. The openings 

 upon the surface of the body are called Spiracles or Stigmata, 

 and are either oval or made in the shape of a slit, as is 

 seen in the adjoining cut. In the soft-skinned insects they 



Fig. 273. 



Spiracle of Common Fly. 



are surrounded by a ring of cartilage, to prevent their closing 

 by ordinary accidents or pressure, and all spiracles are pro- 

 tected by a kind of sieve or grating, made up of hairs ex- 

 tending from either side of the aperture, which keeps out 

 dust, that would otherwise enter with the air and stop the 

 passage. The interchange of air is effected by the enlarge- 

 ment and contraction of the abdomen. The rings, (or skele- 

 ton), which surround the abdomen, are seldom inflexible, but 

 are made up in one part of membrane, and the horn-like ends 

 are brought together by muscular contraction, by which 

 means expiration is effected. The enlargement or inspira- 

 tion is accomplished by the simple elasticity of the encasing 

 rings of the body, as well as of the trachea. Hence, full- 

 ness is the natural or passive state of the respiratory or- 



What are the spiracles ? How are they sometimes protected? Give the mechanism 

 of respiration. 



