6o THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



the slight depressions on the oral surface of the disc between 

 the pseudo-tracheae. As the drying continues the bubbles run 

 into the interbifid grooves and through them into the pseudo- 

 tracheae, clearly showing that the grooves lead into the pseudo- 

 tracheae. 



T/ie collecting channels. 



It has already been stated that the anterior and posterior 

 sets of pseudo-tracheae run into common collecting channels, 

 and that the central pseudo-tracheae also run into separate closed 

 channels. These channels, which are kept open by incomplete 

 chitinous rings without bifid extremities, communicate with the 

 exterior by narrow fissures which are continuations of the 

 longitudinal fissures of the pseudo-tracheae. Since there are no 

 interbifid spaces there are no interbifid grooves or other openings 

 into these channels. Each channel opens into its corresponding 

 gutter between the prestomal teeth in a remarkable manner, the 

 more deeply situated portions of the proximal rings being 

 expanded and prolonged towards the prestomum, so as to form 

 a spout-like opening to the channel. 



The posterior common collecting channel of one labellum 

 has ten pseudo-tracheae opening into it. Throughout the greater 

 part of its length the extremities of the rings are either quite 

 plane, or slightly expanded, or possess only the rudiments of 

 forks. Consequently the channel opens to the exterior by a 

 longitudinal fissure only. At its proximal end the chitinous 

 bars representing the rings are elongated and form a shallow 

 groove leading towards the discal sclerite which forms the. side 

 of the entrance of the mouth. The central pseudo-tracheae open 

 through their own collecting channels into gutters between the 

 prestomal teeth. In Fig. 22 the proximal portions of three of 

 the central p.seudo-tracheae with their collecting channels ter- 

 minating in spout-like openings are illustrated. 



When at rest the oral surfaces of the labellae or oral lobes 

 are in apposition, but during feeding they are spread out over 

 the surface of the food so as to form an oval disc. In order to 

 attain this position that part of the oral surface of the labellum 

 adjoining the prestomum, which is situated just external to the 



