THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 59 



Though difficult to describe the arrangement can be easily 

 understood by reference to PI, XI 11, fig. i, representing a dissec- 

 tion of a portion of a pseudo-trachea. On the right-hand side the 

 integument of the oral surface of the labellum has been removed so 

 as to show a portion of the pseudo-trachea with the alternate bifid 

 and flattened extremities of the chitinous rings and the membrane 

 lining the interior of the tube stretching between them. On the 

 left-hand lower portion the appearance of the surface integument 

 is represented. Two interbifid grooves leading to their interbifid 

 spaces are shown. Between the interbifid spaces are elevated 

 masses, each of which is produced by a fold of the integument 

 enclosing the flattened end of a ring and the extremities of the 

 adjacent forks on each side. In the left-hand upper portion of 

 the diagram is shown the appearance of these structures as seen 

 by transmitted light so as to indicate the relationship of the 

 integument to the rings. 



In PI. XIII, fig. 4, illustrating transverse sections of four 

 pseudo-tracheae the interbifid groove is indicated on the left side 

 of each pseudo-trachea, but is perhaps best seen in the central 

 ones. In each case the point of bifurcation of the chitinous ring 

 is indicated by a dark spot above which the forks are curved 

 inwards. From the point of bifurcation a distinct line, which 

 represents the reflection of the cuticle at the base of the interbifid 

 groove, passes obliquely upwards and outwards to the surface of 

 the integument. Fig. 20 illustrates diagrammatically the con- 

 dition seen in transverse sections. 



Anthony (1874), Wright (1884) and Lowne (1895, p. 395) all 

 regarded the interbifid grooves as suckers. The latter figured 

 them as blind sacs attached to the forks of the rings with openings 

 into the pseudo-tracheai only. None of these authors seemed 

 to regard them as channels leading into the pseudo-tracheae. 



The fact that these interbifid grooves are really channels 

 leading into the pseudo-tracheae can be demonstrated however 

 by a very simple experiment. If the proboscis of a blow-fly is 

 placed in alcohol, formalin or other preserving agent, and the 

 suctorial disc is later mounted in water under a cover-glass and 

 examined with the aid of a microscope the grooves can be clearly 

 seen. As the specimen begins to dry air bubbles often form ia 



