APPENDIX 231 



food should not be allowed to cover the old food and 

 thereby drown the pupse. 



Precautions. — In most cases a single female 

 With one, or in some cases more males should be 

 put into a bottle, except for stock breeding, when 

 more flies should usually be used. Under favorable 

 conditions 200 to 300 flies should be obtained within 

 the first ten days after hatching has commenced. 

 The parents should be removed about ten days after 

 the beginning to avoid overlapping of generations. 

 If desirable they may then be transferred to a second, 

 or even to a third culture bottle. In summer 

 the parents should not remain over eight days in 

 the old bottle; in winter they may be left more than 

 ten days with safety. To obtain virgin females for 

 mating, the bottles should be thoroughly emptied — 

 it may be necessary to remove the old paper, etc., 

 in order to make certain that all of the old flies are 

 removed — and females obtained not later than six 

 hours after the bottle w^as emptied. Females ob- 

 tained twelve hours after the bottle was emptied are 

 not certainly virgin. If the old males have not been 

 removed females so young as to have their wings not 

 yet unfolded may in rare cases have already under- 

 gone copulation. 



Examination. — The flies go toward the Hght. 

 Therefore, if the bottle is held with its mouth away 

 from the light, the flies are not likely to escape. 

 The plug may then be removed and another smaller 

 bottle, with a mouth that fits the larger one, placed 

 mouth to mouth against the larger bottle. The com- 



