76 THE CANADIAN ENTOMOLOGIST 



1. Dormant. — In cool retreats where suitable shelter and 

 protection may be found; here flies may truly hibernate. 



2. Periodically active. — In premises where an increased tem- 

 perature produces activity in the fly which would otherwise be 

 inactive and dormant. 



3. Permanently active. — The gradation between the former 

 state, and this would be governed by temperature and the pres- 

 ence of food. Permanently active flies have been found by my- 

 self and other observers in every month of the winter season from 

 November to March. I have dissected such flies from Decem- 

 ber to March and found them capable of reproduction in many 

 instances. Such flies are found in warm bakehouses, kitchens, 

 restaurants and stables. Jepson (1909) used such flies for 

 breeding experiments in February. 



4. In the immature stages. — The previous states, Nos. 1 to 3, 

 are based on actual observations. Tha't in northerly latitudes M. 

 domestica may be found in the developmental stages (egg, larva or 

 pupa) is a statement that has only, so far as I know, a theoretical 

 and experimental basis. It should be possible, one would think, 

 to find M. domestica breeding in permanently warm places, such as 

 stables where larval food is present. In many stables, however, 

 the temperatures are very variable, and this fact would lengthen 

 the different stages very considerably. Personally, I have so far 

 failed to discover evidence of M. domestica breeding under natural 

 cojiditions during the winter months in the latitudes of Ottawa 

 (Canada) and England, but. observations indicate the possibility 

 of such an occurrence in the presence of suitable conditions. 



In the light of the evidence at present available, I think we 

 are still justified in regarding the dormant and periodically active 

 states during the overwintering period as the usual occurrence in 

 northerly latitudes. But there is no doubt that where circum- 

 stances render state No. 3 possible, it contributes very materially 

 to an increase in the number of available and active flies early in 

 the spring. I have always held the same view as that suggested 

 by Copeman and Austen (1914): "That the relative lateness of 

 the season at which house-flies annually become abundant may be 

 due to the smallness of the number of individuals that, in an active 

 condition, survive the winter in houses or other buildings." This 



