114 



AN INTRODUCTION TO ENTOMOLOGY 



aquatic larvae the spiracles are closed; in these insects the air is 

 purified by means of gill-like organs, termed tracheal gills. A few 

 insects have blood-gills. 



Two types of respiratory systems, therefore, can be recognized: 

 first, the open type, in which the air is received through spiracles; and 

 second, the closed type, in which the spiracles are not functional. 



a. THE OPEN OR HOLOPNEUSTIC TYPE OF RESPIRATORY ORGANS 



That form of respiratory organs in which the tracheae communicate 

 freely with the air outside the body through open spiracles is termed 

 the open or holopneustic type.* 



As the open type of respiratory organs is the most common one, 

 those features that are common to both types will be discussed under 

 this head as well as those that are peculiar to this type. Under the 

 head of closed respiratory organs will be discussed only those features 

 distinctly characteristic of that type. 



i. The Spiracles 



The position of the spiracles. The spiracles are situated one on 

 each side of the segments that bear them or are situated on the lateral 

 aspects of the body in the transverse conjunctivas. 



The question of the position of the spiracles has not been thor- 

 oughly investigated; but I believe that normally the tracheae, of 



T 



Fig. 130. Lateral view of a silkworm showing the spiracles 

 (After Verson) 



which the spiracles are the mouths, are imaginations of the transverse 

 conjunctivas between segments. From this normal position a spiracle 

 may migrate either forward or backward upon an adjacent segment 

 (Fig. 130). 



The number of spiracles. The normal number of spiracles is ten 

 pairs; when in their normal position, there is a pair in front of the 



*Holopneustic: holo (6Xos), whole; pneuma (iri'fvfj.a.), breath. 



