148 Arthropods as Simple Carriers of Disease 
manure, dirty waste paper, decaying vegetation, decaying meat, 
slaughter-house refuse, sawdust-sweepings, and many other sources. 
A fact which makes them especially dangerous as disease-carriers 
is that they breed readily in human excrement. 
The eggs are pure white, elongate ovoid, somewhat broader at 
the anterior end. They measure about one millimeter (1-25 inch) 
in length. They are deposited in small, irregular clusters, one 
hundred and twenty to one hundred and fifty from a single fly. A 
female may deposit as many as four batches in her life time. The 
eggs hatch in from eight to twenty-four hours. 
The newly hatched larva, or maggot (fig. 108), measures about two 
millimeters (1-12 inch) in length. It is pointed at the head end and 
blunt at the opposite end, where the spiracular openings are borne. 
It grows rapidly, molts three times and reaches maturity in from six 
to seven days, under favorable conditions. 
The pupal stage, like that of related flies, is passed in the old 
larval skin which, instead of being molted, becomes contracted and 
heavily chitinized, forming the so-called puparium (fig. 108). The 
pupal stage may be completed in from three to six days. 
Thus during the warm summer months a generation of flies may 
be produced in ten to twelve days. Hewitt at Manchester, England, 
found the minimum to be eight days but states that larvas bred in 
the open air in horse manure which had an average daily temperature 
of 22.5 C., occupied fourteen to twenty days in their development, 
according to the air temperature. 
After emergence, a period of time must elapse before the fly is 
capable of depositing eggs. This period has been termed the pre- 
oviposition period. Unfortunately we have few exact data regarding 
this period. Hewitt found that the flies became sexually mature in 
ten to fourteen days after their emergence from the pupal state and 
four days after copulation they began to deposit their eggs ; in other 
words the preoviposition stage was fourteen days or longer. Griffith 
(1908) found this period to be ten days. Dr. Howard believes that 
the time “must surely be shorter, and perhaps much shorter, under 
midsummer conditions, and in the freedom of the open air.’’ He 
emphasizes that the point is of great practical importance, since it is 
during this period that the trapping and other methods of destroying 
the adult flies, will prove most useful. 
Howard estimates that there may be nine generations of flies a 
year under outdoor conditions in places comparable in climate to 
