VIII 

 TRACHEAL RESPIRATION 



The tracheal respiration as found within the Arthropoda in 

 the groups Onychophora (Peripatus), Solifuga (Bernard, 1895), 

 Phalangida, some Acarina, Myriapoda, Chilopoda, and Insecta 

 represents a solution of the problem of providing oxygen to the 

 cells, different in principle from the mechanisms so far dis- 

 cussed, a solution which allows a high degree of organization 

 to be attained, but which sets rather narrow limits to the 

 possible size. 



The tracheae are air-filled tubes originating from special 

 openings in the integument, the "spiracles," and by repeated 

 branching reaching out to all parts of the body and oftei. 

 penetrating into the very cells, thus obviating the convection 

 by a circulating fluid. 



There is no hard and fast line of distinction between tracheal 

 and pulmonary respiration. In many Arachnidce we find re- 

 duced tracheal systems acting mainly as lungs supplying the 

 blood (p. 57). Even in the tracheates proper a circulating 

 blood fulfils other necessary functions of distribution and 

 cannot avoid transporting some oxygen and a larger amount 

 of C0 2 . In several insects (Larvae of Hypoderma and Tipulida, 

 Walter, 1 922 ; Gerbig, 1 9 1 3) we find a rich supply of tracheoles 

 floating in the blood and very probably supplementing as a 

 kind of lung the direct supply of the tissues. The supply of 

 oxygen obtainable in this way must be limited, however, 

 because the blood does not contain any respiratory pigment 

 and there can be no separation of "arterial" from "venous" 

 blood. The blood-flow moreover is usually rather slow. It 

 might be worth while, however, to study the oxygen tension 

 and the oxygen-carrying capacity of the blood in the large 

 Tipulid larvae. The C0 2 -carrying capacity of the blood is 

 much larger because bicarbonates are regularly present 



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