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 htiman 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, hke 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 temperattire 

 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 

 midsvimmer 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 



