252 ZOOLOGY FOR MEDICAL STUDENTS , chap. 



after about 5 days in this stage the adult fly emerges. About 7-10 days 

 later the female begins to lay eggs. 



It is during the adult phase that the fly becomes a source of danger. 

 Catholic in its tastes it is at one moment wandering about amongst and 

 feeding on human faeces, at another wading in the jam on a tea-table, 

 at another creeping about food in course of preparation in a kitchen. 

 Human faeces are liable to be crammed with microbes of diseases of the 

 alimentary canal, such as Diarrhoea, Typhoid, Cholera, Dysentery, and 

 such microbes taken into the alimentary canal of the fly are apt to remain 

 alive and even in the case of bacteria to multiply within it. The fly 

 when feeding, especially when feeding on sugar or other soluble material, 

 exudes from time to time a drop of clear fluid from its mouth and both 

 this and the faeces of the fly are liable to infect the food on which they 

 are deposited. Portions of faecal material containing microbes are also 

 liable to adhere to the feet or other parts of the fly and to be left behind 

 when it wades in jam or other food-material. Such disease germs lying 

 in wait in food-material are obviously liable to infect with disease human 

 beings who swallow them. 



It is clear that House-flies constitute a distinct menace to health, 

 and therefore that their numbers should be kept down to the minimum 

 about human habitations. The adult flies should be destroyed by 

 fly-papers, " tangle-foot," ^ or poison.^ But here again, as is the case 

 with mosquitos, the most effective measures are those directed against 

 the earlier stages of the Ufe-history. The all-important thing is to 

 prevent the accumulation of garbage, fresh manure, or faecal matter in 

 places accessible to flies and in the neighbourhood of habitations. Such 

 materials are preferably incinerated : if they have to be accumulated, 

 the accumulations should be buried under at least two feet of earth. 



The genus Fannia (or Homalomyid) includes the small House-fly {F. 

 canicularis) and the Latrine fly {F. scalaris), distinguishable from Musca 

 by their smaller size, the greater overlap of the wings when at rest, the 

 plain (not feathery) bristle of the antenna, and the fact that two nervures 



1 Heat together 62 parts of resin, 26 of castor oil, and 12 of honey. Dip 

 skewers or iron wires in this and leave about. When well covered with flies 

 incinerate them in a flame and recoat the wire. 



^ Sodium arsenite i lb. (or Cooper's Sheep-dip Powder 3J lbs.), sugar 10 

 lbs. (or treacle 2 gals.), water 10 gals. Owing to its very poisonous character 

 it is well to colour this solution with some distinctive dye. Cloths moistened 

 in it may be hung up ; or bottles of it may be left standing about with wick 

 dipping into the solution. 



Another good fly poison consists of 3 per cent Formaline in sweetened 

 milk or water. Leave about in rooms during the night in saucers, taking care 

 that no other fluid is available for the flies to drink, 



