RESPIRATORY APPARATUS OF INSECTS. 



675 



Larva?, nearly every segment is provided with a pair ; but in the 

 perfect Insect, several of them remain closed, especially in the 

 Thoracic region, so 



that their number F IG - 347. 



is often considerably 



reduced. The struc- JBM 



ture of the Spi- ^? SB 



racles varies greatly ^ ^^k^MH 



in regard to com- 

 plexity in different - - 

 Insects ; and even ; /?Q 

 where the general 

 plan is the same, 

 the details of con- 

 formation are pecu- 

 liar, so that perhaps 

 in scarcely any two 

 Species are they jS^^j£ 

 alike. Generally 

 speaking they are ^/ 



furnished with some ? M 



kind of Sieve at 

 their entrance, by 

 which particles of 

 dust, soot, &c, 

 which would other- 

 wise enter the Air- 

 passages, are filtered 



out ; and this sieve may be formed by the interlacement of the 

 branches of minute arborescent growths from the borders of the 

 Spiracle, as in the common Fly (Fig. 348), or in the Dytiscus ; or 



; 



Portion of a large Trachea of Dytiscus, with 

 some of its principal branches. 



Fig. 348. 



Spiracle of Common Fly. 



x x2 



