2l6 TROPICAL MEDICINE AND HYGIENE 



discharges from ulcers, or from the conjunctiva, they 

 have abundant opportunities of picking up germs. 



(2) After feeding, the flies for a period regurgitate the 

 fluid parts ot the food, pus, &c., and with the regurgita- 

 tion are disgorged some of the organisms derived from 

 the filth on which they have fed. 



(3) Defaecation. Flies defaecate frequently when well 

 fed, and the fasces will contain all organisms that have 

 survived the passage down the intestines. 



Probably direct fouling by contact and by regurgitation 

 are the most important. The fluid regurgitated serves 

 to soften substances such as sugar on which they rest, 

 and in swallowing the syrup thus formed some organisms 

 are left behind. There is always the possibility that the 

 larvae have derived some pathogenic organisms and 

 transmitted them to the adult, but this is probably, in the 

 case of M. domestica of minor importance, as many 

 organisms are destroyed in the intestines of the larvae 

 before pupation, and in any case the later stages of 

 larval life are passed in moist earth or decaying vegetable 

 matter. 



Musca domestica is oviparous. The adult lays eggs which 

 hatch out into larvae and maggots. In the case of other 

 flies there are differences. Many of the Sarcophagidae, 

 for instance, are larviparous, and deposit living young 

 larvae on the surface of faeces and other animal matter. 

 These larvae live on and in the faeces, but in cesspits do 

 not remain deep, as the deeper parts contain no oxygen, 

 and are practically anaerobic. The larvae grow rapidly, 

 but remain on the upper layer of the faeces, and as fresh 

 dejecta are dropped on the surface these are rapidly 

 invaded by the large, partly mature, larvae, and devoured. 

 The larvae will feed many times on the same mass, 

 passing it rapidly through their intestines. With large 

 larvae all thin-shelled eggs, such as ankylostomes, are 

 destroyed and many of the bacilli, whilst the faeces are 

 rapidly deodorized, as observed in the Malay States. 

 When the larvae are fully mature they escape from the 



