ORGANS OF BREATHING. 73 



the membranes, penetrate the muscles, and extend 

 through the legs and the wings. 



Two sorts of these air organs have been distin- 

 guished, the one tubular or pipe-like, as those we have 

 just described, and another vesicular or cell-like. 



The first sort, or air pipes (*), are composed of three 

 distinct membranes, the outer and inner of which are 

 thick, extensible, and of a cellular texture, while the 

 middle one is formed of a gristly thread rolled spirally 

 round in the manner of a corkscrew, and very similar 

 to the spiral air pipes of plants. This gristly spiral 

 thread is very elastic, in consequence of which the 

 pipe is kept uniformly open, for even when it is com- 

 pressed by the muscles, it immediately expands again. 

 There is a similar mechanism of gristle in the human 

 wind pipe. 



These air pipes have also been distinguished, from 

 their situations, into arterial and pulmonary, the arte- 

 rial being those which come directly from the spira- 

 cles ; and the pulmonary, the two large pipes on each 

 side of the body, (not always traceable), from which 

 other arterial pipes branch off. The structure of both 

 these is similar. 



The second sort, or air-cells ( 2 ), are without the 

 spiral gristle, being composed only of an outer and 

 inner membrane. Consequently, those air cells, when 

 not filled with air, must become flaccid by their sides 

 collapsing. They are not in the form of pipes, but 

 like cells or pouches, mutually communicating with 

 each other through very short and simple canals. 

 They never commnnicate directly with the spiracles, 

 but receive their air from the air tubes. They are not 

 found in all insects ; but, when they occur, they appear 

 to serve the purpose of reservoirs for air. 



They vary also in number and in size. In the rose 



(1) In Latin, Tracheae tubulariee. 

 (2) In Latin, Tracheae vesicularia. 



