11 



vagina and the anal tract unite at their posterior end 

 forming a common opening. Peculiar sac-like evaginations, 

 filled with "bacteria, occur in the wall of the anal tract 

 near the point of union and open into the lumen. A 

 longitudinal slit in the membrane which separates the 

 anal tract from the oviduct lies immediately opposite the 

 opening of the evaginations. As the eggs pass along the 

 vagina, the surface of each egg is pressed through this 

 slit, against the openings. The "bacteria contained within 

 the evaginations are smeared over the surface of each egg 

 from where they find their way through the micropyle into 

 the egg. The bacteria are then incorporated into the 

 embryoiogic development of the insect. 



The larvae which hatch from the eggs possess four 

 spherical ceca near the fore-part of the mid-intestine. 

 These ceca contain the "bacteria which may also "be found 

 throughout the lumen of the alimentary tract. During 

 the pupal stage a bulo-like diverticulum tranches off the 

 esophagus just in front of the "brain. (A similar struc- 

 ture has "been observed "by Dean (1933; 1935) in the apple 

 maggot, Rhagoletis pomonella , "but its possible relation to 

 "bacteria has not "been determined. ) The "bacteria accumulate 

 in this structure from which they later (in the adult fly) 

 spread throughout the alimentary tract, including the anal 

 sac-like evaginations. From this location the "bacteria . 

 are transmitted to the next generation of eggs and thus 

 they are perpetuated. According to Stammer (1929) and 

 Allen and Eiker (1932), similar "bacterium- insect relation- 

 ships exist in other species of Trypetidae. In Tephritis 

 heiseri a similar aperture exists "between the vagina and 

 hind- intestine "but the latter does not' possess the clavi- 

 form protrusions as does Dacus oleae. Instead, this area 

 of the hind- intestine is differentiated into long drawn- 

 out channels which are narrowed in the direction of the 

 opening. These are filled with "bacteria which are applied 

 to the eggs during oviposition. The freshly laid egg is 

 covered with a layer of mucus in which the bacteria 

 multiply until they enter the egg through the micropyle. 



Among the ticks instances of generation to generation 

 transmission of "bacteria are not so well known. One im- 

 portant case in this regard, however, is the generation 

 to generation and stage to stage transmission of Pasteurella 

 tularensis in the Rocky Mountain wood tick, Dermacentor 

 andersoni . This discovery was made "by Parker and Spencer 

 (192*0. 



