The common house-fly is not so innocent as he looks, but really a dirty little thing. He has not a sharp beak-like proboscis, and cannot stab, but he has a soft, dabbing proboscis, which he pushes on to every kind of filth as well as walking with his six legs on such matter. Then he comes and wipes off minute particles and germs on to our food, our lips, our fingers, and faces. It is quite certain that he, and others allied to him, are thus the means of spreading typhoid fever in camps where there are open latrines and open larders and mess tables. The house-fly breeds from a maggot, just as the blue-bottle or blow-fly does, but very few people have ever seen or recognised the maggot of the house-fly. The reason is that it lays its eggs in horse dung, and the grubs are hatched in the muck-heaps of stables. That is also the reason why it is much less numerous in London than it used to be, since stables and mews are now fewer and cleaner than they were. It is also the reason why the house-fly abounds in ill-kept country inns and farmhouses. Its breeding ground is just outside the window.

There is not only one common house-fly in this country: there are three kinds, in addition to the blue-bottle or blow-fly, which is distinguished at once by its great size and blue colour, and lays its eggs in carrion. Late in the year you may often see what would pass for young or starveling house-flies going about among the others. This is a distinct species, the Homalomyia canicularis of entomologists. The third kind only to be distinguished by careful examination with the aid of a magnifying glass, is Anthomyia radicum. Both these are much less abundant than the common house-fly (Musca domestica), with which they almost always occur. Their breeding habits are similar to those of the common house-fly.

A fourth kind of fly is invariably mistaken for the common house-fly when it is noticed, as it sometimes is, in consequence of the sharp stab which it inflicts. As recently as the beginning of November last year I was “bitten” or pricked by one of this fourth kind in a London club. They are common enough on the sea shore in autumn, and may be a severe nuisance. People generally take them for common house-flies which have lost their temper in the hot weather and give way to the bad habit of “biting” out of sheer exasperation. Really, of course, a house-fly could not stab or prick with its broad-ended proboscis. The fly in question, which looks almost exactly like a well-grown house-fly, but possesses a sharp and business-like beak or proboscis, is known to scientific men as Stomoxys calcitrans. There are many kinds of Stomoxys scattered all over the world, and it is probable, though not actually proved, that they carry parasites such as the trypanosomes of horse and cattle diseases from one animal to another, as do the species of Glossina or tsetse-fly.

But we have yet to learn more about these flies and the parasites they transfer. In the case of the gnat, it has been discovered that the malaria parasite is swallowed by the gnat, and multiplies in it, producing thousands of spores in its blood, and it is these spores which the gnat hands or rather “mouths” on to man. No such multiplication of the trypanosome in the tsetse-fly (Glossina) is known. The tsetse-fly passes on the trypanosome as it received it, and yet it seems as though it is not any and every biting fly which can pass on the trypanosome of nagana, or of sleeping sickness, but only the particular species of tsetse-fly. Perhaps it is a case of greater abundance, the tsetse-flies being the obvious and dangerous carriers of trypanosome disease where they occur, on account of their abundance and the fierceness and celerity of their attack. It is almost certain that in India, Burma, and South America some other flies must transfer the trypanosomes from animal to animal, causing the diseases known as surra and mal de caderas, because no tsetse-flies—that is to say, no flies of the genus Glossina—occur in those countries, and no other mode of transference, except by some blood-sucking insect, seems probable.

Ants in Africa are carriers of infection, and possibly also in London kitchens, where a little red ant sometimes abounds. The black beetle or cockroach is a creature to be got rid of, as it is very probable that it spreads certain kinds of infection over food and dishes during the hours of “revelry by night” which kind-hearted people allow it to enjoy in their kitchens.