240 PARASITES OF FLIES 



{/?) CritJiidia. 



Under the name of Crithidia muscce-doniestiae, Werner ( 1909) 

 and Rosenbusch (1909) have each described a flagellate parasite 

 in the intestinal tract of the house-fly, and more recently 

 Swellengrebel (191 2) has described a species C. calliphorcE in the 

 blow-fly {C. erytJirocepJiala). Patton speaking of the genus says : 



"Crithidia in its adult flagellate stage is a very characteristic organism and can 

 never be mistaken for a Trypanosoma or a Herpetomonas, even in the fresh condition. 

 Its body is pointed at both ends, the anterior (flagellar) end being always drawn out 

 to a fine point ; this end may be of considerable length or it may be short. The 

 posterior end is usually pointed and may be markedly so, or it may be more or less 

 blunt. The nucleus is situated at about the middle of its body and the blepharoplast, 

 usually a large structure measuring as much as i //, is always situated close to the 

 nucleus, either just anterior or a little distance posterior. Arising from it there is a 

 well-marked flagellum which may be marginal or pass along the body depending how 

 the parasite lies. In the majority of forms the undulating membrane is a narrow 

 ectoplasmic band so that the flagellum exhibits very few undulations, in some species 

 however the latter are quite marked.... The flagellates of this genus have a characteristic 

 developmental cycle ; in those cases in which the infection is contaminative the cysts 

 are ingested by the hosts •,...the preflagellate stage is characterised by an increase in 

 growth and possible multiplication by simple fission. In the next, the flagellate, 

 stage, the flagellum develops at the margin of the parasite and instead of projecting 

 freely is attached to the body by a narrow undulating membrane, as the flagellum 

 becomes free, the anterior end of the parasite is drawn out. The flagellates multiply 

 by simple longitudinal division or by multiple rosette formation. Owing to the 

 irregularity exhibited in the method of divison flagellates of all sizes are produced. 

 After remaining an indefinite time in the intestines they pass down and encyst in the 

 rectum, and are then passed out in large numbers in the fteces. In the case of those 

 CritJiidia that are transmitted hereditarily the flagellates pass to the ova into which 

 they penetrate and then round up." 



The presence in the intestinal tracts of biting-flies of parasites 

 belonging to the genera Herpetomonas and CritJiidia has been 

 the cause of considerable difficulty in the elucidation of the ways 

 in which such flies transmit trypanosomes, pathogenic parasites, 

 which in some respects resemble them. 



(2) Nematoda. Habronenia niiiscce. 



Carter (1861) appears to have been the first to describe a 

 parasitic nematode worm in the house-fly. He found that in 

 liombay about half the flies were infected ; the worms numbering 

 two to twenty being found chiefly in the proboscis and head. 

 He called the worm Filaria niiisae and described it as follows : 



