34 



ELEMENTARY ENT( )M( )LOGY 



abdominal segments but the last two or three are small openings 

 called spiracles, or stigmata, which are the external openings 

 through which air is admitted to the trachea. The spiracles are 

 guarded by hairs and other devices, to prevent the ingress of dust 

 and foreign matter, and each has a valve operated by a special muscle 

 which opens and closes it. From each spiracle a short tube extends 

 inward and opens into a main tracheal tube which extends along 

 the side of the body. There are commonly two of these main tubes, 



or tracheal trunks, on either side of 

 the body, which give off three main 

 branches in each segment. The upper 

 branch goes to the dorsal muscles, the 

 middle one branches to the alimentary 

 canal and reproductive organs, and the 

 lower one supplies the nerve cord and 

 ventral muscles. These branches divide 

 and subdivide into the finest tubes, 

 which penetrate all the tissues, run- 

 ning between the muscle fibers ; some 

 authorities state that they may even en- 

 ter individual cells. They do not end 

 blindly, but anastomose so as to form 

 a capillary network, so that a contin- 

 uous circulation of air is possible. By 

 opening the spiracles the air enters 

 the tracheal system, and it is expelled 

 by muscles which cause a vertical con- 



FIG. 39. Diagram of tracheal traction of the bod walls and thus 

 system in body of a beetle . > 



force it out. The rhythmic expansion 



sp, spiracles ; /;-, trachea. (After J 



Koibe) and contraction of the body occurs at 



a regular rate, dependent upon the 



temperature and the activity of the insect, and resembles the 

 breathing of higher animals. Many insects are provided with 

 large air sacs which serve as air reservoirs. The trachea are 

 readily recognized by their striated appearance, which is due to 

 a thickening of the cuticle into a thread, which lies on the inner 

 surface in a compact spiral, like a compressed spiral spring, and 

 thus prevents the collapse of the tubes. 



