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364 C. GOKDON HEWITT. 



weeks or months during the summer and autumn, and in the 

 case of the females have deposited many batches of eggs ; 

 their life work, therefore, is complete. Those flies which 

 hibernate are, I believe, the most recently emerged, and 

 therefore the youngest and most vigorous. On dissection it 

 is found that the abdomens of these hibernating individuals 

 are packed with fat cells, the fat body having developed 

 enormously. The alimentary canal shrinks correspondingly 

 and occupies a very small space; this is rendered possible by 

 the fact that the fly does not take food during this period. 

 In some females it was found that the ovaries were very well 

 developed, while in others they were small, and mature 

 spermatozoa were found in the males. Like most animals in 

 hibernating, M. domestica becomes negatively heliotropic 

 and creeps away into a dark place. In houses they have been 

 found in various kinds of crevices such as occur between the 

 woodwork and the walls. A favourite place for hibernation 

 is between wall-paper which is slightly loose and the wall. 

 A certain number hibernate in stables, where, owing to the 

 warmth, they do not become so inactive, and they emerge 

 earlier at the latter end of spring. During the winter the 

 hibernating flies are sustained by means of the contents of 

 the fat body, which is found to be extremely small in hiber- 

 nating flies if dissected when they first emerge in May and 

 June. The abdominal cavity is at first considerably decreased 

 in size, but the fly begins to feed and soon the alimentary 

 tract regains its normal size, and, together with the develop- 

 ment of the reproductive organs, causes the abdomen to 

 regain its normal appearance. The emergence from hiber- 

 nation appears to be controlled by temperature, as one may 

 frequently find odd flies emerging from their winter quarters 

 on exceptionally warm days in the early months of the year 

 (see Appendix). 



3. Flight. 



The distance that M. domestica is able to fly is one of 

 practical importance in connection with their breeding habits 



