120 RESPIRATORY MECHANISMS 



essentially from that of larger insects, a fact which appears to 

 be correlated both with the small absolute size and with the, 

 even relatively, small capacity of the tracheal system. 



Most of -the spiracles remain closed when no processes re- 

 quiring extra oxygen are in progress in the corresponding 

 segment, but the third thoracic and eighth abdominal main- 

 tain a regular rhythm opening slightly and closing completely 

 several times per minute. This rhythm is affected both by 

 lack of oxygen and by excess C0 2 . At a C0 2 pressure of 15 

 mm (2% in air) the spiracles remain permanently open. 

 Wigglesworth shows that there are no local receptors sensitive 

 to CO2, and his analysis of a large number of experiments 

 leads to the conclusion that the acidity of the tissues is the main 

 regulating factor. This contention is proved, to my satisfac- 

 tion at least, by a very ingenious experiment in which a flea 

 was enclosed under the microscope in a small air space sur- 

 rounded by alkaline pyrogallol absorbing both C0 2 and 2 . 

 In these conditions which involve the production of lactic acid 

 in the tissues all the spiracles remained permanently open, 

 and on readmission of air after 20 minutes asphyxia it took 

 several minutes before any of them began to close, and 20 

 minutes before the rhythm was normal. 



In the hedgehog flea, Herford (1938) found that the oval 

 main tracheae would begin to collapse during each period of 

 closure, showing that the closing mechanism is air-tight. 



The closing mechanism and its reaction to C0 2 in low con- 

 centration was demonstrated even in a tick, Ornithodorus, by 

 K. Mellanby (1935). 



The oecological significance of the diffusion regulation is 

 to be sought, as maintained by Hazelhoff and confirmed by 

 Wigglesworth, in the reduction of water loss. The exchange 

 of gases is not reduced, but will require a steeper concentration 

 gradient when the passage is obstructed and, provided the 

 atmosphere inside the tracheal system remains saturated with 

 water vapour, the diffusion of this vapour to the outside and 

 the consequent loss of water is definitely reduced. Investiga- 

 tions by Buxton (1930), Gunn (1933), and K. Mellanby (1935) 



