158 Journal of Agricultural Research voi. xiii. no. 3 



sequent exposures. In no case was there failure to obtain eggs within 

 two or three days after exposure of the moist bran. The eggs so ob- 

 tained were, of course, removed from the building in order not to inter- 

 fere with the normal course of events. 



The foregoing obser/ations on deposition were supplem.ented by a 

 study of a number of females captured from time to time during the 

 winter. They were first examined to determine whether or not they 

 were freshly emerged. This is done by exerting a little pressure on the 

 sides of the thorax with a pair of forceps. In the case of very young 

 flies this causes the ptilinum to extrude, even if it had previously been 

 completely retracted. In the course of some other experiments it was 

 found that in flies exposed to rather low temperatures the ptilinum may 

 retain this elasticity for as long as 12 days, but at the temperature of 

 this animal house it is probable that the ptilinum can not be forced out 

 after the flies are three or four days old. When such pressure does not 

 cause the ptilinum to extrude, it is certain that the fly is past this early 

 stage of plasticity, but hov/ much older it may be is, of course, unknown. 



After this examination the flies were dissected and the spermathecae 

 were examined for the presence or absence of spermatozoa, and the size 

 and development of the ovaries was studied. A regular procedure was 

 adopted in this study, as follows: The abdomen was removed from the 

 freshly killed fly, and from this the entire reproductive system was re- 

 moved. This was put on a hollow ground-glass slide and covered with 

 physiological salt solution. The spermathecse were then removed and 

 mounted on a plain glass slide in salt solution and examined under the 

 compound microscope (4-mm. objective). A little pressure on the cover 

 glass will cause the chitinous capsule to break open and the presence or 

 absence of spermatozoa is easily determined. Plate 7, figure G, gives an 

 idea of the appearance of the spermathecae just after they have been thus 

 broken open. The spermatozoa in lively wriggling masses are seen 

 issuing from the cracks of the capsule walls and also from the severed 

 ends of the ducts. The lines of the figure which represent the sperma- 

 tozoa are relatively darker and heavier than they actually appear under 

 the microscope. 



The ovaries were fixed in Carnoy's fluid and washed in alcohol and 

 then stained in carmalum or borax carmine. After destaining and 

 dehydrating, the ovarioles were separated by careful manipulation with 

 needles and mounted in balsam. Sometimes a little picric acid was 

 added to the 95 per cent alcohol during the dehydrating process in order 

 to bring out the chitinous structures more clearly. The ovaries were 

 found in all stages of development. Camera-lucida drawings of certain 

 well-marked stages were made. These are shown in Plate 7, figures A 

 to F. They are drawn to the same scale, the No. 10 eyepiece and 

 the i6-mm. objective being used. Most of the figures show the struc- 

 tures in optical section, but the surface markings have been represented 



