FLIES AND DISEASE 225 



mandibles for tearing and burrowing into food material. At the oppo- 

 site end the main tracheal trunks open by three slits. It feeds on 

 decomposing refuse. 



It is full-grown in five days in the tropics, in England one to eight 

 weeks . 



The larvct then becomes barrel shaped, skin hard, becomes dark in 

 colour, forms the pupa, this stage lasting about five days. 



Period of development in the tropics about five days. 



Period of development in Itngland about two to ten weeks. 



An adult flv lives from three to sixteen weeks. 



TO SUPPRESS THEM. 



House refuse, street sweepings and all rubbish to be burnt or 

 deposited several miles away from human dwellings and leeward of 

 prevalent winds. 



Latrine refuse should be buried beyond reach of the flies. 



Horse dung required for agricultural or other purposes should be 

 dumped at a distance, treated with c{uicklime, moved about with a fork 

 frequently so that the birds may get at the larvae. Carts conveying 

 refuse mtist not drip their contents on the highwa}'. Some enemies 

 of the fly should be encouraged, as : fungi, e.g., Empusa muscat, 

 spiders, centipedes, larvae of beetles, ants, wasps, toads, lizards and 

 rats. 



The use of gauze and fly-papers adds to one's comfort but are use- 

 less for suppressing flies. 



HOW THEY INFECT MAN. 



The mouth parts of this fly are so made that the}^ cannot get solid 

 particles into their mouths. They taste all foods by pressing their oral 

 lobes upon them. If such food is satisfactory a little fluid is regurgi- 

 tated on to it from its crop in the attempt to dissolve some of the food 

 substance so that it can jjass along the minute pseudo-tracheae. They 

 pour upon the substance, later eaten by man, numerous typhoid bacilli 

 from infected faeces and thus infect a new victim by the food, sugar, 

 bread, milk, meat or edge of the cup, &c. Not only so, but they 

 defascate about fifty times in twenty-four hours and the typhoid bacilli 

 pass out perhaps in an enhanced condition of virulence and infect the 

 food materials, water and utensils of the human host. These were very 

 instrumental in spreading typhoid fever during the Boer War, igoo- 

 1902. 



The blue-bottle or blow-flv is of the same family (Genus Calliphora) 

 and has similar morphology and life history. 



The fruit-fly Drosophila ampelophila is also a musca with a similar 

 history. 



15 



