THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 383 



equal to nearly 129 mm. of mercury between the external and 

 the internal nitrogen pressure, which would sufficiently account 

 for the transmission of the nitrogen from the blood, in which 

 the tension of nitrogen would equal the higher pressure, through 

 the cuticular epidermis, to the atmosphere, by transfusion. 



The objection that the passage of the dissolved nitrogen 

 through the blood would be very slow is not a valid one, for 

 although it would be undoubtedly very slow in a stationary 

 fluid, there is no reason to believe that it would be slow in a 

 circulating fluid, like the blood of an insect, which is alternately 

 exposed to the tracheal tubes and the inner surface of the 

 skin, and therefore to alternate differences of nitrogen pressure. 

 Moreover, a considerable quantity of the tracheal air escapes 

 during flight, by the expiratory sound-producing spiracles, and 

 this air must have the composition of the intra-tracheal air ; so 

 that the percentage of nitrogen in the tracheal tubes is always 

 tending to that in the external air, as fresh air is being pumped 

 into the tracheae to maintain the total pressure. Hence the 

 supposed difficulty due to the increase of the nitrogen, in the 

 tracheae, vanishes. 



Movements of the Tracheal Vessels. Rhythmic vibrations of 

 the tracheal vessels have been observed, in the elytra of 

 Coleoptera (Landois [150]) and in the legs of fleas (Furlonge).* 

 These are readily accounted for by variations of pressure in 

 the vessels. Landois considers them to be due to currents 

 of blood, but this explanation is unsatisfactory, and could 

 not possibly account for the rhythmic expansion and contrac- 

 tion of the vessels which I have myself seen in the leg of the 

 Flea (Pulex). Such movements show rhythmic variations of 

 the intra-tracheal pressure ; and the wave-like movements of 

 the finer tracheae in the elytra of Coleoptera, if they result from 

 changes of pressure, would greatly assist in the absorption 

 of the included gases by the blood, by bringing a fresh layer of 

 the circulating fluid into contact with the tracheal wall. 



It is easily shown that the agitation of a fluid in contact 



* Furlonge, on Pulex irritans. Journal of Quekett Mic. Club, ist series, 

 vol. iii., pp. 189-203. 



26 



