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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 pupa3 through the winter 

 as I have been able to keep the pupae of Stomoxys 

 calci trans 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 larvae 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 spermathecae, which facts support this 

 view. Griffith (1. c.) has succeeded in rearing batches of 

 eggs 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 egg- 

 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 



