120 



ENTOMOLOGY 



thoracic and eight abdominal; but in adults the prothoracic pair is 



almost always suppressed. (See page 60.) 



Though tracheae are absent in most Collembola, Sminthurides 

 aquaticus has tracheae in the head, which open through 

 a pair of spiracles on the posterior part of the head, 

 there being a spiracle on each side of the neck. Two 

 other species of Collembola, Sminthurus fuscus and 

 Actaletes neptuni, are likewise known to have such a 

 tracheal system, limited to the head. 



The spiracles are usually provided with bristles, 

 hairs or other processes to exclude dust; or the hairs 

 of the body may serve the same purpose, as inLepidop- 

 tera and Diptera; in many beetles the spiracles are pro- 

 tected by the elytra; but in other beetles and in many 

 Hemiptera and Diptera the spiracles are unprotected 

 externally. Larvae that live in water or mud may have 

 FIG. 175. Larva spiracles at the end of a long tube, which can be 



of Bittacomor pha . , ..,.. r .,1 T j 



davipes, showing thrust up into the pure air ; this is true of the dipter- 

 respiratory tube ous i arvae o f Eristalis, Bittacomorpha (Fig. 175) and 



Natural size. After 



HART. Culex (Fig. 232). 



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 



FIG. 176. 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; 5, 

 spiracle; /, trachea. After JUDEICH and NITSCHE. 



a circular muscle, but Coleoptera and Lepidoptera, among other'insects, 

 have a somewhat complex apparatus for closing the trachea immediately 

 behind the spiracle. Thus, in the stag-beetle, a crescentic bow (Fig. 

 176, b) extends half around the trachea, and the rest of the circumfer- 

 ence is spanned by a lever (/) and a band (bd) ; these three chitinous parts, 



