BESPIRATOKY APPARATUS 



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distributed in the elongated labium or ' tongue ' of the % (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, and 

 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' (g), through 

 which the air enters and is discharged ; whilst they give off branches 

 to the different segments, 

 which divide again and 

 again into ramifications of 

 extreme minuteness. They 

 usually communicate also 

 with a pair of air-sacs (A) 

 which is situated in the 

 thorax ; but the size of 

 these (which are only found 

 in the perfect insect, no 

 trace of them existing in 

 the larvae) varies greatly 

 in different tribes, being 

 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 office : for within 

 the membrane that forms 

 their outer wall an elastic 

 fibre winds round and 

 round, so as to form a 



spiral closelv resembling Fl <*- 741. Tracheal system of Nepa (water- 

 in its nom'tinn ami fnno scorpion) : a, head ; 6, first pair of legs ; c, first 



segment of thorax ; d, second pair of wings ; e, 



tions the spiral wire spring second pair of legs ; /, tracheal trunk ; g, one 

 of flexible gas pipes : with- of the stigmata ; h, air-sac, 

 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 

 tracheal system has been dissected out, and so pressed in mounting 

 that 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 



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