(e) The presence of larvae in the alimentary canal of man is by no means uncommon. Both the larvae of Musca and Fannia are not infrequently found; and over a thousand of the latter have been passed by a highly infected individual at one time. They probably make their way into the body with over-ripe fruit. In some cases they give rise to no symptoms, but in others violent pains are felt and a certain dizziness, and the digestive functions are interfered with. The presence of these larvae in the urinary passages is even more difficult to explain, but they undoubtedly are at times found in these channels.
A few years ago an elaborate investigation was carried on by Mr. W. Nicol, for the Local Government Board, on the part played by flies in the dispersal of the eggs of parasitic worms. He showed quite definitely that the ova of certain human parasites are taken into the fly and pass through its body undigested. Should these be deposited on the food of man, there is great risk of his becoming infected. As I have said before, flies take only liquid food, and it is only when the ova of the parasites are very small that they can pass into their alimentary canals. Some eggs are too large for the fly to swallow. Eggs of parasitic worms have also been shown to be carried on the legs and proboscides of flies, and these are deposited on the spot where the fly next cleans itself. Probably, however, in the end little harm is really done by flies in disseminating parasitic worms, but it is a possibility which must not be altogether disregarded.
The remedial measures for the control of flies are fully dealt with in Graham-Smith’s admirable book, ‘Flies in Relation to Disease,’ from which I have ventured to borrow many figures; and again by Dr. Gordon Hewitt, in his work on ‘House-flies,’ which has had such a wonderful success in stimulating our North-American cousins to decrease the numbers of one of the gravest enemies to mankind.
It has been shown over and over again that we can control the mosquito: the building of the Panama Canal alone proves this. We could equally control the ‘Infinite Torment of Flies.’ The Canadians and Americans are doing their best; but are we? The knowledgeable world has at least discovered the reason why Beelzebub was called the ‘Lord of flies.’
CHAPTER VII
MITES
Part I
THE HARVEST-MITE (Trombidium)
Natura in minimis maxime miranda.
(Linnaeus.)