STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 503 
as much as six weeks, the greater time being spent in the pupal 
state which was sometimes of three or four weeks duration. 
In no case was I able to keep the pupx through the winter 
as I have been able to keep the pupe of Stomoxys 
calcitrans and other forms. 
My experiments and observations point to the fact that in 
the presence of suitable larval food, such as excremental 
matter or decaying and fermenting food materials in a moist 
and warm condition, the female flies would lay their eggs 
and the larvee would develop if the temperature of the air 
were sufficiently high for the prolonged activity of the flies. 
In winter this last condition is not naturally satisfied, but 
under such conditions as are found, for example, in warm 
restaurants and kitchens, stables and cowsheds, female flies 
may be often found during the winter. On dissecting such 
flies I have found mature ova in the ovaries, and living 
spermatozoa in the spermathece, which facts support this 
view. Griffith (l.c.) has succeeded in rearing batches of 
eges in November, December, and early January under 
artificial conditions, which further proves their ability, given 
the necessary conditions as regards temperature, to breed 
during the winter months. In this country M. domestica 
breeds, as a rule, from June to October, and the greatest ege- 
laying activity prevails in August and September. As I have 
already contended, and as Griffith has shown, they may breed 
at other times if the necessary conditions are present ; I have 
obtained eggs from flies caught in restaurants in December ; 
Keller also mentions the fact that he obtained eggs in January. 
These facts may account for the rapid appearance of flies in 
the early summer. It is not unlikely that the flies which 
survive the winter months, which many spend in a dormant 
condition if they are not fortunate enough to remain active 
in a warm restaurant or stable, lay their eggs, almost 
immediately on renewing their activity, in such places as 
manure-heaps which are kept, as is often the case in towns, 
under cover, and which are consequently warmer externally 
than those in the open. In this way a large number of flies 
