STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 539 
forward, and is in association with the corresponding pro- 
thoracic and meso-thoracic ventral discs. The smaller and 
more posterior disc (d. mt.) will develop into the remaining 
portion of the much reduced meta-thoracic segment, including 
the halteres. 
. Reference has already been made to other imaginal rudi- 
ments which occur in the abdominal region as circular patches 
of embryonic cells. The abdominal segments develop from 
numerous segmentally arranged plates of a similar nature, 
which are found during the early pupal stage. 
During pupation the imaginal rudiments increase in size 
and are not destroyed by the phagocytes in histolysis, as is 
the case with most of the larval structures. The cephalic 
discs are evaginated by the eversion of their saes by way of 
the anterior end of the larva, a cord of cells attached to the 
dorsal wall of the anterior end of the pharynx marking the 
path of eversion. A similar process takes place in the case 
of the thoracic imaginal discs, which, by their eversion, build 
up the whole of the skeletal case of the thorax and its dorsal 
and ventral appendages, the wings, halteres and legs. 
VI. Summary. 
1. An account of the previous work on the breeding habits 
of M. domestica is given, which, together with the author’s 
investigations, show that the house-fly breeds in the following 
substances : 
Horse-manure; this is preferred by the female flies as a 
nidus for the eggs, and forms the chief substance in which 
they breed ; human excrement, either in the form of isolated 
feeces or occurring in such places as latrines, privies and ash- 
pits; cow-dung; poultry excrement; also in substances con- 
taminated or mixed with excremental products, such as 
bedding from piggeries and from rabbits and guinea-pigs, 
paper and textile fabrics which have been contaminated, as 
cotton and woollen garments, sacking, rotten flock-beds, 
straw-mattresses, cesspools ; decaying vegetable substances, 
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