IV. IXSECTA 



401 



The respiratory organs are trachea: (figs. 437, 438) which resemble 

 the trachea of man only in that they are tubes filled with air, and kept 

 from collapse by firm walls. They open to the exterior by openings 

 (spiracles, stigmata) on the sides of the body. They are inpushings of 

 the skin and consequently have the same structure, an epithelium and an 



outer chitinous layer. The latter lines 

 the lumen of the tubes, and since it must 

 be thin to permit the passage of gases 

 (oxygen, carbon dioxide), and at the 

 same time firm, to keep the tubes open, 

 it is thrown into folds which usually 

 pursue a spiral course. The turns of the 

 spiral are so close that it gives the tubes a 



FIG. 437. FIG. 438. 



FIG. 437. Tracheal system of Machilis (f rom Lang, after Oudemans). k, head; 

 I-III, thoracic somites; s, spiracles; i-io, abdominal somites. 



FIG. 438. Portion of trachea of caterpillar (from Gegenbaur). .-1, main trunk; 

 B, C, D, branches; a, epithelium with nuclei, b; d, air in tracheal tube. 



ringed appearance (fig. 438). Inside the spiracles the trachea* branch 

 repeatedly until they end in fine tracheal capillaries in the tissues. 

 In general it may be said that each segment has a pair of spiracles 

 and corresponding tracheal systems (tig. 60), but this scheme is 

 complete in no known species, for there are always some segments 

 (especially in the head) which lack these organs and are supplied from 

 adjacent segments (fig. 437). Again, the trachea? may be connected by 



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