364 
C. GORDON HEWITT. 
weeks or mouths 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. domestic a 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 eai’ly months of the year 
(see Appendix). 
3. Flight. 
The distance that M. domes tic a is able to fly is one of 
practical importance in connection with their breeding habits 
