THE ANATOMY OF INSECTS. 



39 



These spiracles are cither simple openings into the respiratory 

 , m, or arc provided with valves, sieves, or fringes of hair for the 

 exclusion of dirt. They lead into a system of air-tubes termed tra- 

 The accompanying figures will indicate the distribution of 

 the main trunks of these tracheae in a cockroach. There is a short 

 trunk arising from each spiracle ; these are all connected together 

 by a large longitudinal trunk on each side of the body, and by 

 numerous tfansverse trunks. From these large tracheae there arise 

 a great number of smaller ones, not shown in the figures, which 

 branch and subdivide, and extend to all parts of the body. When 

 one dissects an insect the viscera are found to be connected together 

 by the ramifications of these tracheae, so that in order to remove 

 any organ it is necessary to cut some of them. The smaller 

 branches of the tracheae are exceedingly minute, and are intimately 



..Tracheal system of Cock- 

 roach. The dorsal integument re- 

 moved and the alimentary canal 

 in place. (After '' nny.) 



Fie. 51. Tracheal system of Cock- 

 roach. The alimentary canal re- 

 moved to show the ven 

 cheal communications. 

 Miall and Denny.) 



ciated with the various tissues. By means of these fine tracheal 

 trunks the air is carried to the various tissues; so that tl sup- 



plied with oxygen directly from the air. without the intervention of 

 blood as in the higher animals. 



minute structure of the tracheae has been the source of 

 much discus-iM. lint at last it seems to be well understood. 

 walls of the trachea; are composed of three layers, which correspond 

 to the layers of the body-wall. In fact the tracheae are looked upon 



tions of the body-wall. The continuity of th- 

 branes of the tracheae and body-wall is shown dia : , vally in 



54. It should be ol that it is the inner layer of the wall 



be trach orrespondswith the outer layer of the wall of the 



