The House-Fly and Its Near Relations 



eggs are elongate, white, and hatch very soon after being laid, in 

 six or eight hours. The larvae, which are white, pointed mag- 

 gots, as shown in the accompanying figure, grow rapidly, cast 

 their skin twice, and reach full growth under favorable conditions 

 in four or five days. The outer skin then hardens, swells out, 

 turns dark brown in color, and within it the true pupa is formed. 

 In this stage it may live for five days and the adult fly issues at 

 the expiration of this time through a round hole in the anterior 

 end of the pupal covering. This makes the total life round for a 



Fig. 96. Musca domestica. (Author's illustration.') 



single generation in summer approximately ten days. Thus 

 there is abundance of time for the development of twelve or 

 thirteen generations in the climate of Washington every summer. 

 The number of eggs laid by an individual fly averages about 

 1 20 and the enormous numbers in which the insect occurs is thus 

 plainly accounted for, especially when we consider the abundance 

 and universal occurrence of appropriate larval food. The univer- 

 sal occurrence of uncared for piles of horse manure in cities is 

 therefore not only a source of great discomfort but is inimical to 

 health since the house-fly undoubtedly distributes disease germs. 

 The numbers in which house-fly larvae occur in horse manure 

 piles may be understood when the statement is made that from 

 a quarter of a pound of manure from the centre of a pile 160 

 larvae and 146 puparia of the house-fly were taken. This would 

 make about 1,200 house-flies to the pound of manure. This is 

 not a fair average, but indicates possibilities and is an actual 

 record of an individual case. Experiments conducted by the 



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