136 



ENTOMOLOGY 



Spiracles. The paired external openings of the tracheae 

 occur on the sides of the thorax and abdomen, there being 

 never more than one pair to a segment. Though the thysa- 

 nuran Japyx has n pairs, no winged insect has more than 10; 

 although there are in all 12 segments which may bear spiracles 

 the three thoracic and the first nine abdominal segments. 

 (Additional details are given on page 66.) 



The spiracles, or stigmata, are usually provided with bris- 

 tles, hairs or other processes to exclude dust ; or the hairs of 

 the body may serve the same purpose, as in Lepidoptera and 

 Diptera; in many beetles the spiracles are protected by the 

 elytra; in other beetles, however, and in many Hemiptera and 

 Diptera the spiracles are unprotected externally. Larvae that 

 live in water or mud may have spiracles at the end of a long 



FIG. 173. 



,. m 



Apparatus for closing the spiracular trachea in a beetle, Lucanus. A, trachea 

 opened; B, closed; b, bow; bd, band; c, external cuticula; /, lever; m, muscle; s, 

 spiracle; t, trachea. After JUDEICH and NITSCHE. 



tube, which can be thrust up into the pure air ; this is true of 

 the dipterous larvae of Eristalis, Bittacomorpha (Fig. 172) 

 and Culex (Fig. 229). 



Closure of Spiracles. As a rule, a spiracle is opened and 

 closed periodically by means of a valve, operated by a special 

 occlusor muscle. In dipterous larvae the closure is effected by 

 the contraction of a circular muscle, but Coleoptera and Lepi- 

 doptera, among other insects, have a somewhat complex appa- 

 ratus for closing the trachea immediately behind the spiracle. 



