DISTRIBUTION OF TRACHEJE. 



571 



FIG. 290. 



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 c 

 again into ramifications 

 of extreme minuteness. 

 They usually communi- 

 cate also with a pair of air- c 

 sacs (h) which are situated e 

 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 g 

 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 

 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 modification of this 

 structure, the purpose of which 

 is not apparent; for instead of 

 its tracheae being kept pervious 



r>, .-i i > T* v .i.T~ Portion of a large Trachea of Dytiscus, with 



after the USUal fashion, by the some of Us principal branches. 



Tracheal system of Nepa (Water-scorpion): a, 

 head; 6, first pair of legs; c, first segment of the 

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

 legs;/, trachea! trunk; g, one of the stigmata; h, air-sac. 



their 



FIG. 291. 



