THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 55 



Lovvne (p. 390) describes the mouth as " a cyHndrical tube 

 extending from the thecal (or discal) sclerites to the prepharyn- 

 geal tube, which may be regarded as its posterior Hmit or isthmus 

 faucium." The deep cleft between the two lateral halves of the oval 

 disc into which the mouth opens he terms the prestomum (p. 143). 



The dissections and feeding experiments described later 

 show that the liquid food is sucked into the pseudo-trachefe and 

 drawn through the collecting channels and along the gutters of 

 the prestomum into the mouth, and it seems probable that crop 

 contents and saliva may be forced at will in the reverse direction 

 for distribution over solid food which has to be moistened and 

 dissolved. In order to explain the process of sucking food into 

 the mouth the structures involved., namely the oral surface of 

 the suctorial disc, the pseudo-trachese, and the prestomal cleft, 

 must be described in detail. 



The pseudo-tracheae, varying in number between 28 and 32, 

 run transversely across the labellum or lobe of the "oral sucker. 

 They form three sets. The seven anterior pseudo-trachese run 

 into a common longitudinal collecting channel which opens into 

 the prestomum between the first and second prestomal teeth, 

 and the posterior eight to twelve in the same way run into a 

 common posterior collecting channel, which opens into the 

 prestomum at its shallow posterior extremity. The central 

 pseudo-tracheas terminate in short channels which run directly 

 into the prestomum without the intervention of common collect- 

 ing channels. By this arrangement all the pseudo-tracheae are 

 made to converge to the prestomum. The arrangement of the 

 pseudo-tracheae is clearly shown in PI. XIV, fig. i, which is a 

 photograph of the oral surface of the expanded suctorial disc of 

 a blow-fly with 30 pairs of pseudo-tracheae. On each side the 

 anterior eight run into a common anterior collecting channel, 

 and the posterior twelve into a common posterior collecting 

 channel, while the ten central pseudo-tracheae are continued 

 separately into the prestomum. 



From the points at which they cease to be tubular in structure 

 the collecting channels are continued along the prestomal cavity 

 to the mouth as grooves or gutters, whose lateral walls are formed 

 by the prestomal teeth. 



