THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 57 



The really effective entrances into the pseudo-tracheae are 

 through the spaces between the bifid extremities of the rings 

 and not through the narrow continuous zigzag fissure, which is 

 at any time extremely narrow and is probably closed during the 

 act of feeding, as will be explained later. 



The pseudo-tracheae gradually diminish in diameter as they 

 approach the margins of the disc, and the size of the forked 

 extremities of the rings and consequently of the spaces between 

 them also diminishes though not to a corresponding degree. 



The term ' interbifid space' is used to indicate the area 

 enclosed between the forks of the bifid extremity of a ring. 



Fig. 21 illustrates two consecutive rings with their bifid and 

 flattened extremities, and Fig. 20 illustrates a side view of one of 

 these rings. PL XIII, fig. 3, is a photograph of several consecutive 

 pseudo-tracheal rings which have been treated with potash and 

 compressed. The terminations of the consecutive rings are well 

 shown. PI. XIII, fig. 2, is a photograph of the oral surface of the 

 disc of a blow-fly after treatment with potash showing portions of 

 four pseudo-tracheae. The longitudinal fissures of the pseudo- 

 tracheae and the forked extremities of the rings and interbifid 

 spaces can be clearly seen. PI. XIII, fig. 4, is a photograph of a 

 section of the disc showing four pseudo-tracheae cut transversely. 

 The chitinous rings, the openings of the longitudinal fissures 

 and the ridges caused by the projection of the tubes above the 

 surface are clearly shown. 



The pseudo-tracheae of several of the common non-biting 

 flies closely resemble each other, though they exhibit slight and 

 apparently unimportant diff'erences in their structure. The 

 average measurements of the various parts in six common species 

 are as follows. 



