68 THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



lobes of the suctorial disc are pressed together so that the lumen 

 of the prestomal cavity is obliterated, and no food can enter the 

 mouth except through the collecting tubes. The pseudo-tracheae 

 are channels kept open by chitinous rings situated in their walls. 

 Each ring has one bifid extremity, enclosing between the horns 

 the ' interbifid space,' which forms an opening of definite size 

 into the pseudo-trachea. A fold in the cuticle, the ' interbifid 

 groove,' leads to each interbifid space. 



The fluid food is sucked first along the interbifid grooves 

 through the chitin-lined interbifid spaces into the pseudo-tracheae. 

 Particles of larger diameter than the interbifid spaces ('006 mm.) 

 are usually prevented from entering the mouth and are rejected. 

 The fluid and smaller particles are drawn along the pseudo- 

 tracheae, through the collecting channels and gutters between 

 the prestomal teeth into the mouth. By means of strong suction 

 two opposite interbifid grooves may be made to communicate 

 with each other owing to the lateral fissures connecting with the 

 longitudinal pseudo-tracheal fissure being forced open, and 

 consequently a few larger particles, up to "02 mm. in diameter, 

 ma}' be drawn into the pseudo-tracheae. 



Certain relatively large and very attractive objects, such as 

 the ova of tape-worms, too large to pass through the filter, may 

 occasionally be swallowed. Such objects probably pass directly 

 into the mouth, when the prestomal cavity is open, during the 

 prolonged sucking efforts made by the flies. 



The large number of experiments which have been made 

 leave little room for doubt that under natural conditions, 

 especially when the fly is feeding on a thin film of moisture, the 

 filtering apparatus works with a high degree of efficiency. 



CHAPTER VI 



THE FUNCTIONS OF THE CROP AND PRO\'ENTRICULUS 



" The structure and function of the crop and proventriculus 

 are matters of considerable interest in considering the distribution 

 of infectious material by flies. At the commencement of a meal 



