INSECT " PORTERS " OF DISEASE 



215 



to deposit their eggs. Their habits are diurnal, but they 

 prefer shady places, and at night are quiescent. Their 

 power of flight is considerable, especially when aided by 

 the wind, but when they reach a suitable resting-place 

 they do not travel far from it. 



In England the great breeding places are the huge 

 deposits or middens of household refuse which disgrace 

 the outskirts of many of our towns, fig. 91. From such a 

 breeding place the flies after hatching out can travel half 



FIG. 91. Municipal rubbish heap. 



a mile or more, but will enter the first rows of houses they 

 meet, and will not then travel much farther. The width 

 of two or three streets is sufficient to provide an effective 

 barrier to the invasion, though the houses, and particularly 

 provision shops, in the first and second streets will be full 

 of flies. 



The adult fly carries germs, mainly bacilli, in three 

 ways : (i) The proboscis and limbs become fouled with 

 the excrement or other material on which they rest and 

 feed, and as these include any ulcerated surface, any 



