250 THE HOUSE FLY—DISEASE C ARRIER 
of which lack legs. The leap is made by bringing the 
two ends of the body together and suddenly releasing 
them, like a spring. In this way it will sometimes jump 
three or four inches. The species is cosmopolitan at 
present, and it was doubtless originally imported from 
Europe into the United States in old cheeses. 
Careful observations have been made on the life his¬ 
tory of this fly by several writers. In 1892 Miss Mary 
E. Murtfeldt studied the life history of the summer 
generation in a western packing establishment. She 
found that the eggs were laid in rather close clusters 
of from five to fifteen, and were also deposited singly. 
About thirty seem to have been laid by a single female. 
The egg is white, slender, oblong, slightly curved, one 
millimeter in length, and with a diameter about one- 
fourth of its length. It hatches in about thirty-six 
hours, and the larva completes its growth in from seven 
to eight days, reaching a length of seven to nine milli¬ 
meters. Where food is sufficient the larva does not 
move about, and groups of them will sometimes com¬ 
plete their growth in the same crevice in which the 
mother fly deposited her eggs. When full grown, how¬ 
ever, the larva moves away to some dry spot, contracts 
in length, assumes a yellowish color, and gradually 
forms into a golden-brown puparium four or five milli¬ 
meters in length. The adult fly issues in ten days. 
Thus three weeks may complete the entire life cycle, 
in August, in St. Louis. 
In Europe, Kessler found that the average summer 
duration of this insect is four to five weeks, and states 
