18 



APPLIED ENTOMOLOGY 



drawn into lungs and there being taken up by the blood and carried to 

 the parts of the body where it is needed, as in man, is carried directly 

 to those parts by a system of air tubes which open along the sides of 

 the body (Fig. 25) . Here the air enters the tubes and proceeds through 

 them to where it is utilized. The openings by which the air enters are 



called spiracles, and these occur in pairs 

 on some of the thoracic and most of the 

 abdominal segments, varying somewhat 

 in number and in position on the seg- 

 ment in different insects. The spiracles 

 often have valves by which they can be 

 more or less completely closed at will. 



Each spiracle opens into a short tube 

 or trachea which, with the others of 

 that side, soon joins a similar tube run- 

 ning along the side of the body and 

 quite close to its surface. From these 

 longitudinal tracheae, branches pass off 

 in various directions, and in turn branch 

 again and again until every part of the 

 body is reached by its air supply. The 

 tracheae frequently enlarge here and 

 there, forming so-called air sacs. 



The tracheae are lined by chitin con- 

 nected .with that of the surface of the 

 body. In these tubes, however, it is 

 formed with spiral thickenings which act 

 like a spring, keeping the tracheae open 

 when not under pressure. There is 

 probably considerable pressure on them 

 in different places by the movements of 

 various parts of the body in walking 

 and other activities, as well as by regular 

 respiratory movements, and the resulting 

 temporary variations in diameter aid in 

 the circulation of air in these tubes. 

 Not only are the tracheae of use in carrying oxygen to all parts of the 

 body, but they also receive the carbon dioxid gas produced by the activi- 

 ties of the cells and permit it to escape through the spiracles from the 

 body, thus performing both of the functions which the blood, so far as 

 gases are concerned, accomplishes in man. Blood then, in insects, does 

 not (except in a few cases perhaps) have a respiratory function. 



The destruction of insects by fumigation is accomplished by the sub- 

 stitution of a gas destructive to life, for the air, and this gas enters the 



FIG. 25. Diagram showing ar- 

 rangement of the main tracheal tubes 

 in an insect, a, antenna; b, brain; 

 I, leg; n, nerve cord; p, palpus; s, 

 spiracle; st, branch from main lateral 

 trunk, t, to spiracle; v, ventral branch; 

 vs, visceral branch. (After Kolbe, 

 from Folsom.) 



