378 THE TRACHEAL SYSTEM OF THE IMAGO. 
Kraepelin pointed out, the flow of salivary secretion from the 
lingual glands is due to intra-thoracic pressure. 
During flight the hypopterygia and sacculi are kept distended 
(erected) with air, notwithstanding its continuous escape, by 
which the humming sound emitted is produced. 
Such an intra-tracheal pressure would be sufficient to account 
for the elasticity of the limbs and wings of the living insect, as 
the distended tracheal tubes would aid greatly in supporting 
its weight on its long slender legs, and would give the requisite 
rigidity to its wings in flight. Such a pressure would be suffi- 
cient to enable the insect to support a weight equal to that of 
its own body, with the extended proboscis; the strength of this 
organ when exserted indicates a positive air-pressure equal to 
about 50 to 75 mm. (two or three inches) of mercury, a con- 
clusion at which I arrive by an estimate of the tracheal surface 
of the expanded oral disc. Such a pressure would be main- 
tained by a very moderate compression of the air in the main 
trachez, a diminution of its volume by one-tenth. 
The entrance of the air into the finest tracheal capillaries, 
which do not exceed ty to 2 in diameter, whilst some are of 
immeasurable fineness, is a difficulty which can be more readily 
explained by a positive pressure in the larger air-trunks. 
Wistinghausen* has remarked that the finest capillaries are 
rapidly emptied after death, either by the elasticity of their walls, 
or by the absorption of fluid, so that it appears as if they are 
kept full during life by the intra-tracheal pressure. 
Expiratory Pressure—The received view of expiration in 
insects presupposes that the contraction of the body cavity 
would drive air from the smaller to the larger air-tubes; as 
both would, however, be subjected to an equal increase of 
pressure, no such movement could occur. It is not clear, there- 
fore, how an expiratory effort could effect a change of air in the 
tracheal vessels, even of the third or fourth magnitude. I have 
sought in vain for any evidence of an expiratory current from 
the spiracles. I formerly tried to extract the air from the 
trachee and replace it by immersing insects in fluid and 
* Zeitsch. f. w. Zool., Bd. 49, p. 565. 
