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 larvæ 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 tuned the _preoviposition_ 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 Washington. The number may be considerably increased in warmer climates. The rate at which flies may increase under favorable conditions is astounding. Various writers have given estimates of the numbers of flies which may develop as the progeny of a single individual, providing all the eggs and all the individual flies survived. Thus, Howard estimates that from a single female, depositing one hundred and twenty eggs on April 15th, there may be by September 10th, 5,598,720,000,000 adults. Fortunately, living forms do not produce in any such mathematical manner and the chief value of the figures is to illustrate the enormous struggle for existence which is constantly taking place in nature.