RESPIRATION 67 



for gas analysis by Krogh's micro-method. The following 

 table indicates the results obtained in percentages : — 



Mean 1*9 15-7 82*4 4*2 5*4 90*4 



In a second series of experiments the animals were placed 

 in a mixture of pure O2 and CO2 till no nitrogen remained in 

 the tracheae. The requisite time previously determined by 

 trial was found to be about 10 minutes. Analysis showed that 

 about one minute after being allowed to breathe ordinary air 

 the normal nitrogen content of the tracheae is restored. Even 

 after one second there is 2*5 per cent, of nitrogen in the tracheae, 

 while half a minute suffices to bring the percentage of nitrogen 

 up to 62'5. From the first set of data it is clear that oxygen 

 disappears and carbon dioxide increases in the tracheal tubes 

 during enforced respiratory activity. From the second it is 

 clear that gaseous diffusion takes place within the tracheal 

 system with a surprising rapidity, when one considers the 

 internal friction encountered by the gases in passing along the 

 lumen of tubes of such minute dimensions. If the rhythmical 

 movements of the abdomen which have been supposed to 

 facilitate the renewal of air in the tracheae are truly acts of 

 inspiration and expiration, it seems, from the rate at which these 

 succeed one another, that the air within the tracheal system is 

 renewed to the extent of about twenty per cent, at each move- 

 ment. 



An admirably thorough investigation of tracheal respira- 

 tion in insects has recently been carried out by Buddenbrock 

 and Rohr (1923). The species employed in their researches 

 was Dixipus morosus, the familiar stick insect, whose barrel- 

 like configuration renders it specially suitable for some forms 

 of manipulation. Using Krogh's microrespiration methods, 

 they first demonstrated the fact that closure of the orifices 

 (stigmata) of the tracheal system reduces respiratory exchange 



