.RESPIRATORY APPARATUS 



995 



distributed in the elongated labimn or tongue ' of the fly (fig. 739'). 

 Their general distribution is shown in fig. 741, where 'we see two 

 long trunks (/) passing from one end of the body to the other. :md 

 connected with each other by a transverse canal' 



in every segment ; 



these trunks communicate, on the one hand, by short wide passages 

 with the ' stigmata,' spiracles,' or ' breathing pores ' (a), through 



1*1,1 I I't.T , 1 *1 ^ 1 



whilst they give oft branches 



lischarged 



greatly 

 being 



which the air enters and is ( 

 to the different segments, 

 which divide again and 

 again into ramifications of 

 extreme minuteness. They 

 usually communicate also 

 with a pair of air-sacs (/>) 

 which is situated in the 

 thorax; but the size of 

 these (which are only found 

 in the perfect insect, 110 

 trace of them existing in 

 the larva;) varies 

 in different tribes, 

 usually greatest in those 

 insects which (like the bee) 

 can sustain the longest and 

 most powerful flight, and 

 least in such as habitually 

 live upon the ground or 

 upon the surface of the 

 water. The structure of 

 the air-tubes reminds us 

 of that of the 'spiral 

 vessels ' of plants, which 

 seemed destined (in part 

 at least) to perform a 

 similar oniee : for within 

 the membrane that forms 

 their outer wall an elastic- 

 fibre winds round and 

 round, so as to form a 

 spiral closely resembling 

 in its position and func- 

 tions the spiral wire spring 

 of flexible gas pipes : with- 

 in this, again, however. 



there is another membranous wall to the air-tubes, so that the spire 

 winds between their inner and outer coats. When a portion of one 

 of the great trunks with some of the principal branches of the 

 trachea! system has been dissected out, and so pressed in mounting 

 thar the sides of the tubes are flattened against each other (as has 

 happened in the specimen represented in fig. 742), the spire forms 

 two layers which are brought into close apposition, and a very 

 beautiful appearance, resembling that of watered silk, is produced 



3 s 2 



PIG. 741. Tracheal system of Xt/m i \vatn-- 

 scorpioni : </, head ; b, first pair of legs ; c, first 

 second pair of wings ; e, 

 /', tracheal trunk; tj, OIK- 



of thorax ; 



second pair of legs ; 



of the stigmata; /;, air-sac. 



