RESPIRATORY SYSTEM. 



315 



the body is composed of a more regular and uniform series 

 of segments, being more numerous than the spiracles of the 



imago. In the former they 

 amount to eighteen, that is, 

 nine pairs; whereas, in the 

 generality of perfect insects, 

 the number is, or appears to 

 be, diminished; moreover, in 

 the latter, the spiracles, ow- 

 ing to the greater develope- 

 ment of external covering, 

 are generally more or less 

 concealed. Spiracles are of 

 two kinds, simple or compo- 

 site. In the former the aper- 

 ture is a simple orifice, guard- 

 ed only by hairs, which pre- 

 vent the passage of any fo- 

 reign substance which might 

 be injm'ious : but in the com- 

 posite spiracle the aperture is 

 closed by two horny valves, 

 which move backwards and 

 forwards like a pair of folding 

 doors at each inspiration. 

 The former sort of spiracles 

 are generally found at the 

 sides of the abdomen, where the upper and lower arcs of 

 each segment are united together. The composite spiracles 

 are, on the contrary, appropriated to the thorax. 



The tracheae, which originate in the spiracles, are con- 

 nected with the two great longitudinal air-tubes above men- 

 tioned, which likewise emit an infinite number of ramifica- 

 tions, extending to all parts of the body, like the branches 



Fig;. 150, Respiratory apparatus of Nepn 

 cirierea, in which the only pair of spi- 

 racles are placed at the sides of the 

 anus, the lateral spiracles beinj; obsolete 

 or rudimental. 



