THE FLY. 57 



that the central trunk gives oiit numerous branches, 

 which convey the hlood to the different members of 

 the body. 



Intimately connected with the circulating system 

 is that of respiration; and when you consider that 

 the Fly is formed to be a denizen of the atmosphere, 

 and that its specific gravity is necessarily one of the 

 most important features iu connexion with its exist- 

 ence, you will not be surprised to hear that the appa- 

 ratus by which the air is admitted and allowed to 

 circulate through the body, is very beautiful, and 

 displays marked evidences of a designing hand. If 

 you examiue the under surface of the My with a 

 pocket-lens, you will find extending along the middle 

 of the abdomen a membranous band that divides the 

 homy rings of which the covering is composed into 

 two parts, and at each side of this membranous divi- 

 sion (but on the rings themselves) you will perceive a 

 row of minute punctures which penetrate the so-called 

 chitiue integimient of the body. These little aper- 

 tures, a pair of which is situated upon each abdominal 

 ring, and two pairs upon the thorax, in contiguity with 

 the members, have been variously denominated 

 spiracles, stigmata, or breathiag-holes, and through 

 them the air is admitted iuto the body ; the respira- 

 tory operation is effected in a somewhat similar 

 manner to the same process in the higher animals, 

 namely by muscular dilatation and contraction, in 

 this case of the abdomiaal region. 



Through the spiracles, which we shall find to be 



d5 



