398 THE ALIMENTARY CANAL OF THE IMAGO. 



injection of air into its interior. Gleichen wrote : ' The exten- 

 sion of the proboscis probably arises from the air which is 

 driven by the fly into the base (rostrum], and from this into the 

 middle tube (haustellutri). and finally into the lips.' He further 

 added : ' The fly can, indeed, drive air between the membranes 

 of the proboscis into the lips, but he cannot take in air with 

 them.' Dimmock [68, p. 46], says : ' That it is air, at least 

 for the most part, and not fluid, which expands the inner 

 surface of the labella;, is easily proved by carefully pressing the 

 head of a fly between the fingers until the proboscis is fully 

 distended, and the labella; fully inflated ; then, putting the fly 

 under water, and pricking the inner surfaces of the labellae 

 with a needle, they at once collapse, bubbles of air escaping at 

 the same moment from the opening made in their surfaces by 

 the needle.' Beside these notices, I have found nothing further 

 on the subject ; but in transverse sections of the oral sucker, 

 made from a proboscis distended by dropping an insect into 

 hot alcohol, the inflated air-channels are not equivocal. 



I have represented a portion of a section in Fig. 50, /, and it 

 will be seen that each of the pseudo-tracheal channels forms 

 the outer wall of two air-channels or modified tracheae which 

 separate the hypodermis from the cuticle of the disc. These 

 channels all radiate from the azygos air-sac already men- 

 tioned (see Fig. 51, c), and have the same distribution as the 

 pseudo-tracheae which they accompany. 



Even when sections are made of a flaccid proboscis, it is not 

 difficult to distinguish the air-channels, although their size is 

 greatly diminished. In the pupa when the proboscis has arrived 

 at the stage of development represented in Fig. 32, the separa- 

 tion of the hypoderm from the cuticle of the disc is very 

 apparent ; the intervening space, like the lumen of the tracheae 

 of the pro-imago generally, is filled with a watery fluid which 

 is readily distinguished from the granular blood in the blood- 

 sinuses beneath the hypoderm. 



The air in these channels, when the organ is erected, may 

 be assumed to be compressed. My reasons for this assumption 

 have been fully discussed in the section devoted to the tracheal 



