18 THE HOUSE FLY DISEASE CARRIER 



The Egg 



The eggs are minute and glistening white, and they 

 are all long ovoid in shape. In length they vary from 

 one-sixth of an inch to a little longer. They are laid 

 in clusters of small size and irregular shape, either on 

 their ends or on their sides. Seen under a high power of 

 the microscope, the polished surface appears to be cov- 

 ered with minute hexagonal markings such as is seen 

 in what the histologist calls pavement epithelium. Each 

 female fly lays on the average 120 eggs, or perhaps 

 more, at a time and may lay several times. Forbes' s 

 assistants in Illinois found that eggs from a single fly 

 vary from 120 to 150 in each deposit and that as many 

 as four deposits may be made, or say, 600 eggs by a 

 single fly (in lit.). One hundred and twenty was the 

 number observed by the writer to be the average num- 

 ber, but Doctor Hewitt has counted as many as 150. 



The duration of the egg stage, as observed in Wash- 

 ington, was usually eight hours; that is to say, eight 

 hours after it was laid the egg hatched. These ob- 

 servations were made in midsummer and have not been 

 repeated at other times of the year. Mr. Newstead in 

 Liverpool found that the eggs hatched in periods vary- 

 ing from eight hours to three or four days, the average 

 time being about twelve hours. But he noted that when 

 laid in fermenting materials the incubation period was 

 reduced to a minimum of eight to twelve hours. In 

 a temperature of from 75 F. to 80 F. they hatched 

 in from eight to twelve hours; in a temperature of 60 



