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 ina 
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 trachez to maintain the total pressure. Hence the 
supposed difficulty due to the increase of the nitrogen, in the 
trachez, 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 trachez 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, rst series, 
vol. iii., pp. 189-203. 
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