142 RESPIRATORY MECHANISMS 



in total pressure which will allow the partial nitrogen pressure 

 in the system to be in equilibrium with the tension of dissolved 

 nitrogen in the surrounding water. There is, and must be 

 normally, 1 a permanent relative vacuum in a tracheal system 

 supplied by gills. By a number of very ingenious experiments 

 and analyses Koch demonstrated the rigidity of the tracheal 

 walls in Aeschna nymphs and measured the total pressure of 

 the enclosed gases, which he found to be about 90% of the 

 atmospheric and practically independent of the depth at 

 which the animals were living. 



The indispensable rigidity of the whole tracheal system pre- 

 cludes the existence of any mechanical ventilating arrange- 

 ments and leaves only diffusion as the mechanism for dis- 

 tributing the oxygen absorbed through the tracheal gills. 

 Calculations, based on measurements of the tracheal trunks, 

 and analyses made by Krogh (1920) show that an oxygen 

 pressure difference of about 40 mm will be required to trans- 

 port the necessary oxygen to the thorax and head from the 

 gills in the rectum. This makes it easy to understand that, 

 in spite of the high perfection of the. tracheal gill system, the 

 dragon fly nymphs begin to suffer from oxygen lack when the 

 tension in the water sinks below 55 mm. It is clear also that 

 the large species of dragon flies come close to the size limit 

 attainable with this type of tracheal system. 



In many cases tracheal gills are sufficiently rigid to allow 

 the necessary uptake of oxygen from moist air, but in dry air 

 they lose too much water. It is interesting that in some in- 



1 When the water becomes supersaturated to any considerable extent the 

 pressure in the closed tracheal system will rise above the atmospheric, and 

 several forms breathing through tracheal gills are provided with safety valves 

 in the form of spiracles allowing any excess pressure to blow off into the water. 

 Koch (1934) has utilized this in a most ingenious way to measure the relative 

 importance of the caudal lamellae in Agrion pulchellum nymphs. Placing such a 

 nymph in water, presenting a slight supersaturation of CO2, and opening the 

 tracheal system, he measured the interval in seconds between bubbles being 

 released, before and after removing the caudal lamellae. In one case the average 

 interval in the intact larva was 64 seconds, while after removal of the three 

 lamellae it rose to 1 15 seconds, indicating that in this case 45% of the total respira- 

 tion could take place through the caudal lamellae. 



By a judicious application of this method several disputed problems concern- 

 ing the respiration through tracheal gills proper and the general surface could 

 no doubt be solved. 



