538 THE POPULAR SCIENCE MONTHLY 



mentioned in that the organism which causes the disease must live for 

 a time in the body of some other animal to undergo certain definite 

 changes before it can again induce the disease in another individual. 

 The most important insect-borne diseases belong to this type, for in the 

 case of man and domestic animals, certain insects and ticks act as the 

 secondary host animals for the organisms of many diseases. Thus, 

 yellow fever is spread only through the agency of a certain mosquito, 

 for in its body alone can the yellow fever organism live and undergo the 

 changes that are necessary before it can be introduced into another 

 patient by the bite of an infected mosquito. Malaria belongs to the 

 same category, for it spreads only through the bite of certain mosquitoes 

 that obtain the organisms with their meal of blood, and then after- 

 wards inject into the blood of another person, a later stage of the 

 malarial parasite which has developed meanwhile within the mosquito. 



Diseases in which certain insects act as specific carriers are most 

 numerous and prevalent in the warmer parts of the world, although 

 temperate regions are by no means free from diseases of this kind, which 

 may be referred to as the inoculative type. 



Among insects which disseminate pathogenic organisms without any 

 specific association, the common housefly is without doubt the most 

 important. The rank of the fly in this unenviable profession is due to 

 several facts in its life-history which render it eminently suited to act 

 as a vector for several diseases such as typhoid fever, diarrhoea, dysen- 

 tery, summer complaint of children, etc. The eggs of the house fly are 

 laid preferably in horse manure, upon which the larval stages or 

 maggots feed, but human excrement serves equally well, and when 

 exposed is very likely to provide food for a brood of fly maggots. Less 

 than two weeks are required for the larvae to mature, and after a short 

 resting stage of two or three days the adult flies emerge from their bed 

 of filth. At this time they may quite possibly bear in their alimentary 

 tract bacteria derived from their larval food. Usually, however, they 

 are quite clean when hatched, in spite of the surroundings whence they 

 have come. They do not long remain so, however, for they feed upon 

 animal and human dejecta of all sorts, garbage and other fermenting 

 material, and if still hungry invade markets or houses, where they may 

 leave upon food any bacteria brought upon their bodies, legs, mouth 

 parts or wings. In addition, their excrement deposited as " fly specks " 

 may contain virulent pathogenic bacteria, if they have had access to 

 matter from which these could be obtained. 



This, in brief, is the status of the house fly as a disease carrier and 

 it is readily seen why filth diseases are the ones naturally spread by this 

 insect. Attracted to the nursing bottle or to the baby's mouth they may 

 infect him with bacteria of any of the numerous enteric troubles known 

 as summer complaint. If they have had access to the dejecta of a person 



