Apr. 15, 1918 



Overwintermg of the House Fly 



159 



on some of the larger cells. These six stages were selected arbitrarily as 

 standards of reference, and all the other ovaries were compared with them 

 and grouped according to the stage which they most closel)' approached. 

 The results of this study are given in Table III. 



Table III- — Showing presence of spermatozoa and the stage of development of the ovaries 

 in house flies taken during the winter in a warm building, Bethesda, Md., iQiy 



o Indicates ovaries of flies from which spermatozoa were absent. 

 P.«. Indicates those in which ptilinum could be extruded. , 



b Two eggs lodged in oviduct showing that deposition had recently occurred, 

 c Mature egg lodged in oviduct, showing that deposition had recently occurred. 



Table III shows that of the 59 females examined during the period 

 from January 10 to March 21, 1917, 46, or 78 per cent, contained living 

 spermatozoa in the spermathecae, while in 13, or 22 per cent, the sperma- 

 thecae were empty. In 6 flies, or 10 per cent, the ovaries were in the first 

 stage of development. Correlated with this was the entire absence of 

 spermatozoa and the fact that in 3 of them a slight pressure on the 

 thorax caused the ptilinum to extrude. This proves conclusively that 

 they were very young flies. Of the 1 1 flies, or 18.6 per cent, with ovaries 

 in the second stage of development, spermatozoa were found in only 3 

 cases, and the ptilinum could be extruded in i case. They were but 

 slightly older than those of stage i. With one exception (a fly with 



