DISTRIBUTION OF TRACHEiE. 



671 



short wide passages, with the "stigmata," "spiracles," or breath- 

 ing-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 communi- 

 cate also with a pair of air- 

 sacs (A) which are situated 

 in the thorax ; but the 

 size of these (which are 

 only found in the perfect 

 insect, no trace of them 

 existing in the larvse) 

 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 ves- 

 sels" of Plants, which 

 seem destined (in part at 

 least) to perform a similar 

 office (§ 232) ; 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 functions the 

 spiral wire-spring of flexible 

 gas-pipes; within this again, 

 however, there is another mem- 

 branous wall to the air-tubes, 

 so that the spire winds between 

 their inner and outer coats. The 

 tongue of the Fly presents a 

 curious modiflcation of this 

 structure, the purpose of which 

 is not apparent; for instead of 

 its tracheae being kept pervious 



P, ,1 1^1*1 j_i Portion of a larg^e Trachea of Z?yitsc«s, with 



after the usual laShlOn, by the some of its principal branches. 



Tracheal sys-tem of Nepa (Water-scorpion): — a, 

 head J &, first pair of legs; c, first segment of tlie 

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

 legs;/, tracheal trunk; g', one ofthe stigmata; A, air-sac. 



Fig. 291. 



