THE ANATOMY OF INSECTS. 



39 



These spiracles are either simple openings into the respiratory 

 system, or are provided with valves, sieves, or fringes of hair for the 

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

 chea. 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 transverse 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 



Fig. 52. — Tracheal system of Cock- 

 roach. The dorsal integument re- 

 moved and the alimentary canal 

 in place. (After Miall and Denny. i 



Fig. 53.— Tracheal system of Cock- 

 roach. The alimentary canal re- 

 moved to show the ventral tra- 

 cheal communications. (After 

 Miall and Denny.) 



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

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

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

 blood as in the higher animals. 



The minute structure of the tracheae has been the source of 

 much discussion. But at last it seems to be well understood. The 

 walls of the tracheae are composed of three layers, which correspond 

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

 as invaginations of the body-wall. The continuity of the mem- 

 branes of the tracheae and body-wall is shown diagrammatically in 

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

 of the trachea that corresponds with the outer layer of the wall of the 



