80 HABITS 



are to be found in the poorer quarters, where decaying substances 

 and excrement lie exposed and covered with flies. 



The common house-fly and the raven fly {M. corvind) and 

 other species annoy animals by settling on the more exposed 

 portions of skin, and sucking the secretions from the skin and 

 mucous surfaces. Human beings working or resting out-of-doors 

 are annoyed in the same way. Flies are particularly attracted 

 to open wounds and ulcers and to pus or other pathological 

 secretions on dressings. Even in temperate climates disease-pro- 

 ducing bacteria may be carried occasionally from one individual 

 to another in this way. Whatever may be the significance of this 

 mode of conveyance in temperate climates, it is obviously of far 

 greater importance in the tropics, where insects are present in 

 greater profusion throughout all seasons of the year. In Egypt 

 and the Sudan, according to Sandwith (1904), flies "often alight 

 on food, when coming direct from filth. They also crawl 

 about the face and mouth of human beings and are most 

 persistent in this, evidently in search of moisture." The natives, 

 both adults and children, exhibit remarkable indifference to flies 

 walking over their faces and eyes and there are good reasons 

 for believing that many cases of ophthalmia are produced in 

 this way in Egypt and other countries. Nicholls (1912, p. 85) 

 believes "that the majority of cases of yaws in the West Indies 

 are caused by the inoculation of surface injuries " by a small fly 

 known as Oscinis pallipes (PI. XVII, fig. 4). 



In tropical countries where the lower classes are often 

 unacquainted with the use of latrines, swarms of flies are bred in 

 the faical deposits ; some of these are found upon food, and in 

 dry weather numbers will be found flying around pools and 

 water supplies, and can be observed alighting at the edge of the 

 water to drink. Some of these species for filthy associations far 

 surpass the house-fly, and seem to have become adapted to 

 breeding in human faical deposits, not being found elsewhere. 

 Nicholls (1912, p. 81), in St Lucia, conducted experiments "by 

 exposing human stools in various places on different days for 

 about ten hours, after which it would be found that numerous 

 ova and larvae of flies had been deposited upon them. In all, 

 twenty-five masses were used and approximately 18,000 flies 



