116 



AN INTRODUCTION TO ENTOMOLOGY 



Fig. 131. — Spiracles; a, of the larva of 

 Corydalus; b, of the larva of Droso- 

 phila amoena. 



The structure of spiracles. — In their simplest form the spiracles or 

 stigmata are small round or oval openings in the body-wall. In many 

 cases they are provided with hairs to exclude dust ; in some, as in the 



larva of Corydalus, each spiracle is 

 furnished with a lid (Fig. 131, a); 

 in fact, very many forms of 

 spiracles exist. Usually each spir- 

 acle opens by a single aperture; 

 but in some larvas and pupag of 

 Diptera they have several openings 

 (Fig. 131, 6). 



The closing apparatus of the 

 tracheae. — Within the body, a 

 short distance back of the spiracle, 

 there is an apparatus consisting of 

 several chitinous parts, surrounding the trachea, and moved by a 

 muscle, by which the trachea can be closed by compression (Fig. 132). 

 This is the closing apparatus of the trachea. The closing of this appara- 

 tus and the 

 contraction of 

 the body by 

 the respiratory 

 muscles is sup- 

 posed to force 

 the air into 

 the tracheoles, 

 which are the 

 essential res- 

 piratory or- 

 gans. 



-Diagrams representing the closing apparatus of the 

 trachea?; a, b,c, chitinous parts of the apparatus; m, muscle; 

 A, apparatus open; B, apparatus closed; C, spiracle and 

 trunk of trachea showing the position of the apparatus. 

 (From Judeich and Nitsche). 



2. THE TRACHEAE 



Each spiracle is the opening of an air-tube or trachea. The main 

 tracheal trunk which arises from the spiracle soon divides into several 

 branches, these in turn divide, and by repeated divisions an immense 

 number of branches are formed. Every part of the body is supplied 

 with tracheae. 



In a few insects the group of tracheae arising from a spiracle is not 

 connected with the groups arising from other spiracles; this is the 

 case in Machilis (Fig. 133). In most insects, however, each group of 

 tracheae is connected with the corresponding groups in adjacent seg- 



