37 THE TRACHEAL SYSTEM Ol' 77/A 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 

 tracheat, a diminution of its volume by one-tenth. 



The entrance of the air into the finest tracheal capillaries, 

 which do not exceed ip to 2/i 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. 

 \Yistinghausen* 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 

 trachea: and replace it by immersing insects in fluid and 

 * Xeitsch. f. w. Zool.. I'.d. 4<;. p. 565. 



