THE CANADIAN ENTOMOLOGIST. 77 



idea was supported by my observations in the state of the repro- 

 ductive organs of a portion of the flies found and dissected during 

 the winter months. 



The lasst contribution to our knowledge on this subject is the 

 report of Copeman and Austen, quoted above, on the results of 

 an examination of the species of flies collected in houses, etc., 

 during the winter months. They state: "In conclusion, it would 

 appear that the customary explanation of the perpetuation of the 

 house-fly from year to year has now been fairly tested, and that 

 the evidence obtained fails to support it. If, however, during the 

 season of greatest fly-prevalence, a selection were made of several 

 centres in which house-flies were present in sufificiently large 

 numbers, it would be worth while during the following winter to 

 endeavour to discover whether living pupa could be found in any 

 considerable quantity in the local breeding-places." 



One serious objection prevents my agreement with their 

 conclusion. The evidence contained in their report points to the 

 fact that practically all the specimens of M. domestica that were 

 received were caught in an active condition, and there is no evi- 

 dence submitted to show that any of their correspondents found 

 these flies as a result of searching for them in the hiding places 

 from which Jepson and I have recorded them. In view of this 

 objection the facts submitted by Copeman and Austen cannot be 

 fairly considered as failing to support the explanation usually 

 given. I may say I have repeatedly applied the test they suggest, 

 and in no case have I been able to find either in England or Canada 

 living pupae of M. domestica under outdoor conditions during the 

 winter. Nor has it ever been possible in my breeding experiments 

 in Canada and in England to carry the insect through the winter in 

 the pupal state. In the experiment described in the first part of this 

 paper a special effort was made to find living puparia among several 

 thousand examined, and not a single healthy puparium was discov- 

 ered ; all were either empty or had failed to develop. Had specimens 

 been overwintering in the pupal condition, it is reasonable to expect 

 that living puparia would have been found in the early part of May, 

 as the adults could not have emerged earlier than that date under 

 Ottawa conditions. It is a matter, however, that requires more 

 observations in different localities, but until evidence is secured 



