I 9 I 3" I 9 I 4-] The Common House Fly. 12 1 



puparium or case is at first pale yellow in colour, but soon 

 darkens to bright red, and finally to dark chestnut colour. It 

 measures from 6 to 8 mm. Masses of several dozen pupae 

 may be found at one locality. Under favourable conditions, 

 in from five to seven days the mature fly emerges from the 

 pupa case. If the conditions are cold and unfavourable, 

 fourteen to twenty-eight days may elapse. In order to 

 escape from the puparium, on the anterior surface of the fly's 

 head a frontal sac becomes inflated with air to such an extent 

 as to be equal in size to the fly's head itself. The balloon-like 

 structure pushes off the anterior end of the pupse case, and 

 then the fly struggles out of its barrel-shaped case. Even if the 

 pupa has been buried in manure, this alternate protrusion and 

 emptying of the frontal sac allows of the fly making its way 

 outwards, and once free, the wings, hitherto crumpled up, 

 expand, and the integument hardens, and within one to two 

 hours the insect flies away. In the British Isles, in summer 

 the house fly begins to lay eggs three weeks after the egg, 

 from which itself has sprung, was laid. 



From a quarter to a half mile is a frequent range of their 

 flight, though some have even been found one mile from the 

 place where they were liberated ; but it is probable that want 

 of food or shelter may compel them to fly much greater 

 distances even than this. Where, on the other hand, food is 

 plentiful, flies do not tend to leave the locality, as we only too 

 well know to our cost in the case of the summer lodging. 



Filth of all kinds attracts flies most powerfully ; but ripe 

 fruit, meat raw or cooked, milk, butter, sweets, all attract 

 them. It is easy to see that sweets are often contaminated 

 by filth from the flies. The vast majority of flies which 

 frequent manure-heaps belong to the domestic variety. Flies 

 are particularly attracted to open wounds and sores on 

 animals or human beings, and to the sweat of perspiring 

 animals and men. In sick-rooms it is difficult to keep flies 

 away. It is easy to see how germs of disease may be carried 

 from the diseased to healthy individuals, or to the food which 

 the latter eat. Many experiments have been made, and 

 sterile water has frequently been found to be contaminated 

 with the B. coli as a result of fly infection. 



Flies are attracted by suitability of food, and therefore in 



