THE STRUCTURE AND FUNCTION OF THE PROB(^CIS 6/ 



These objects are extremely attractive to flies, which may 

 suck at segments of tape-worms for several hours in order to 

 extract their contents (Nicoll, 191 1, p. 20). The flies appear to 

 make great efforts to swallow the ova and probably the prestomal 

 cavity is at times open so that the ova pass directly into the 

 mouth without passing through the pseudo-tracheae. This view 

 is supported by the curiously uneven results obtained by Nicoll, 

 who for example in one series of experiments fed seven flies on 

 ruptured segments of T. serrata and found 400 ova in the 

 intestines of two flies, two ova in one fly, and none in the other 

 four flies. The most likely explanation seems to be that in the 

 latter five flies the filter acted efficiently, whereas in the two 

 former ova were allowed to pass into the mouth while the 

 prestomal cavity was open. Possibly in their endeavours to 

 swallow these ova the flies attempt to use their teeth to reduce 

 their size. In order to do so the labellje have to be so widely 

 separated that the prestomal cavity is open, consequently if 

 suction is made during the process of scraping large particles 

 may pass into the mouth. 



All the observations hitherto made indicate that under most 

 conditions the filter acts very efficiently and prevents the 

 entrance of particles larger than '006 mm. in their smallest 

 diameter into the mouth of the blow-fly. Exceptionally a few 

 larger particles may be drawn forcibly through it, or pass directly 

 through the prestomal cavity into the mouth. 



Summary, 



All the non-biting flies examined, C. erythroccpJiala, M. 

 domestica, S. carnaria, F. caniailaris, L. ccesar and 0. anthi'ax, 

 possess a filtering apparatus situated in the pseudo-trachea; 

 of the suctorial disc. The anatomy and action of this filter have 

 been most thoroughly studied in C. erytJuvcepJiala. The suctorial 

 disc is grooved by pseudo-trachese which end near its centre in 

 closed collecting channels. The latter open into furrows or 

 gutters formed by the peculiar disposition of the prestomal teeth 

 on the walls of the prestomal cavity. The opening of the mouth 

 is situated at the base of the cavity. During natural feeding the 



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