Respiration and Metabolism 



267 



TABLE 47 (continued). 



RESPIRATORY RESPONSE TO O. WANT AND CO, INCREASE 



+ =;positive stimulation. — =no effect. ( ) =questionable effect. 



Chortophaga by adding carbon dioxide to the inspired air. In conclusion, the 

 breathing movements of insects and of spiders as welP"'^ respond to the action 

 of carbon dioxide and oxygen on the respiratory centers in the same general 

 way as do the breathing movements of higher vertebrates. 



A further type of respiratory control can be evoked in tracheal organisms 

 by action of respiratory stimulants on the cells regulating the size of the aper- 

 ture of the tracheal ending. These effects have been extensively investigated 

 in the common flea.^^^- •^*'** Tracheal pulsation is accelerated by oxygen defi- 

 ciency and carbon dioxide increase, as well as by temperature elevation in the 

 region of the spiracle. The effect of carbon dioxide and oxygen has been 

 investigated in a number of "ventilation control" insects. The opening of the 

 spiracle in response to carbon dioxide in the cockroach, Periplaneta, is almost 

 instantaneous and occurs at much lower concentrations (2-3 per cent) of the 

 gas than are necessary to stimulate breathing movements of the abdomen (10 

 per cent).^-^^ Exactly what the nature of the spiracle reaction to the respiratory 

 stimulant might be is subject to further investigation and interpretation. 

 Wigglesworth, on the basis of his own extensive investigations and those of 

 other workers, concludes that the action of carbon dioxide is mainly through 

 the acidity produced in the tissue fluid, in other words a hydrogen ion effect. ^^^ 



In this survey of the respiratory control among invertebrates and verte- 

 brates, the importance of carbon dioxide excess and oxygen want is apparent. 

 Many of the findings have been summarized in Table 47, which designates 

 the type of respiratory movement, along with the response to the respiratory 

 gases. In practically all cases in which carbon dioxide increase is a stimulating 

 factor, so also is oxygen want; oxygen deficiency in lower (aquatic) animals 

 appears to be either the only respiratory stimulant or, if carbon dioxide also 

 acts, then the more fundamental one. This implies the absence of carbon 

 dioxide-sensitive respiratory centers in primitive forms and correlates with the 

 fact that carbon dioxide would tend not to build up to such a considerable 

 concentration in aquatic forms as a result of its diffusion through the body 

 surface and away from the organism. Conversely, in higher (generally ter- 

 restrial) animals, carbon dioxide tends to play a more important role as a 

 respiratory stimulant and by its action on the highly sensitive respiratory 



