Page:Popular Science Monthly Volume 81.djvu/43

Rh of each host the parasite undergoes a certain development which is essential to complete the life cycle and insure the perpetuation of the malarial organism. Another kind of mosquito, in the tropics, is responsible in a similar manner for infecting human beings with yellow fever.

With these brief introductory remarks, we will now take up a discussion of the house fly and such other insects as are known to carry disease germs.

The common house fly has been associated with typhoid epidemics so frequently during the past few years that Dr. Howard suggests that it be called the "typhoid fly." Even before the rôle of this insect was definitely understood, it was suspected to have a connection with the disease, because typhoid fever is generally most prevalent in late summer, at the time when flies are the most abundant. I do not wish to convey the impression that typhoid is spread only by means of flies, for such is not the case. There are plenty of other agencies, such as a polluted water or milk supply, but flies play a much more important part in this connection than was supposed a few years ago, and the house fly is more important than other species 'because of its great abundance and its habit of occupying the dwellings of man and crawling over his food.

It was an old idea that flies were not only innocuous but were a benefit to mankind. It is said that Sir John Lubbock provoked a laugh in the House of Commons in 1873 by quoting as follows from one of the books used in the elementary schools: "The fly keeps the warm air pure and wholesome by its swift and zigzag flight."

On the other hand, Kircher, writing in 1658, makes the following statement: "There can be no doubt that flies feed on the internal secretions of the diseased and dying, then flying away, they deposit their excretions on the food in the neighboring dwellings, and persons who eat it are thus infected."

As it took 240 years to demonstrate the truth of this theory, and as twelve years have now passed, and the lesson of the Spanish war has not yet come into general practise, it is a mooted question if, after all, our progress is not an idle boast. At the present day there are many persons who regard flies as a necessary nuisance, but who are not awake to the dangers of their abundance.

As a rule flies do not go far from their breeding places, and if they are very abundant in any locality it is reasonably certain that their breeding place is close at hand.