The house-fly may seem at first much less to be dreaded than any one of the painfully "biting" or (to be correct) skin-piercing and blood-sucking flies; yet it should be regarded as a much greater enemy to humanity and a more dangerous peril than any of those other flies, of which some short mention and description has now been given. Its life-history and its fecundity have been already alluded to; its rapid growth and maturity counterbalance the fact that it is short lived.

From ancient times there has been a consensus of opinion that there was in some way a connection of cause and effect between swarms of flies and the spread of disease. In the plagues of Egypt, in the reign of Pharaoh of the Exodus, it will be remembered how, after "the land was corrupted by reason of the swarms of flies," Exodus viii. 24, there came "a very grievous murrain" upon cattle, Exodus ix. 3, followed by a "plague of boils and blains" upon man and beast.

In our present day insect life is being scientifically investigated with the view of establishing the connection, and of discovering fully the serious rÔle of disseminating disease, of which the house-fly has long been suspect. The microscope reveals much, and the art of bacterial culture now explains how it is true that the superabundant creature, which has persistently followed civilised man into every quarter of the globe, has ever had a share in conveying contagion beyond that of any other household pest.

That the house-fly is bred in filth matters not much. After emerging from the puparium its first voidance of fÆcal matter may be contaminated with live baneful germs, but it voids itself before its first flight. Having six legs it stands upon two pairs, whilst with the other pairs, at one time the front pair and at another time the hind pair, it works frequently and vigorously at brushing and stroking down every part of its body. Though it starts its new life quite a wholesome newborn creature, and though it must be credited with being a most assiduous remover of dirt from its own body, yet from the human point of view its subsequent life is a persistently disgraceful career.

It is the evil course of the newly-hatched and self-cleaned fly not to restrict its diet to the honey of flowers, as do some of its relations. Its food includes excrement, sputum, and every kind of putrefying organic matter likely to be swarming with micro-organisms of a character deleterious to humanity. It is certain that, when only a few days old, a fly will practically abound internally and externally (on the feet) with dangerous germs, as amply proved by methods of laboratory culture. As it feeds, it walks over the food; and the hairy joints of its feet, when microscopically viewed, appear conspicuously liable to carry germs in spite of frequent attempts at self-cleaning. Wherever it alights and walks, it prospects with a touch of its trunk, which is the main instrument of evil. It has a very filthy habit, from time to time, of depositing pale vomit spots as well as dark-coloured fÆcal droppings. These defilements are visible, wherever it may alight on walls, windows, ceilings, and especially on pendent ornaments, whereon the males delight to rest.

Its manner of feeding upon solid food is to pour forth a copious supply of saliva, to regurgitate some previously imbibed fluid draught, and then to re-imbibe; thus, besides devouring soft food, it dissolves, befouls, and feeds on crystalline sugar and other hard dry food materials.

Its regurgitated fluid commonly swarms with bacteria, microbes, and the like. Imbibed bacteria are not inevitably killed in the digestive process of the fly, for its excrement has been found to abound with well recognisable infective germs. In the market, the shop, the larder, and on our tables, the house-fly seeks every opportunity of befouling and contaminating human food.

The varieties of micro-organisms are multitudinous, doubtless many more in number than the microscopist and the bacteriologist expert have yet isolated and registered as capable of identification. Granted that the majority of these are non-pathogenic to humanity, still a formidable number, including some which are very generally disseminated, are virulently pathogenic, and many are suspect. There is no need to give a list of all the infectious diseases which man and beast are liable to contract, but the germs of nearly all may be carried from place to place, from creature to creature, and from person to person, through the intervention or agency of the house-fly. The medical profession are convinced that infantile mortality from epidemic diarrhoea must be attributable to summer flies.

In the matter of food which becomes fly-infected after having been cooked, or of food like milk, butter, and fruit, which are consumed raw, it should be known that a single pathogenic germ of ultra-microscopic dimensions, having obtained lodgment in the body, may there multiply and originate a fatal disease. On the other hand, raw meat which has been infected may, after the bacteria have been all killed by cooking, contain excreted poison in deleterious quantity. The decomposition of infected meat begins ten or twelve hours before the bad odour is perceptible.

Fortunately the omnipresent germs which most commonly deteriorate our food are not very actively deleterious, or are only slightly debilitating; yet wherever such less obnoxious germs get lodgment, there the ready prepared and most favourable breeding place for the worser kind is to be found. The various species of these evil things are not always exterminating competitors; they sometimes flourish in company, and dwell together, like the seven devils within the exorcised and sane man after his relapse, as mentioned in the Scripture.

That food gets fly-blown and maggot-infected is a very disgusting fact, but the plainly visible result is of little hygienic significance apart from the more concealed facts of the fly-borne conveyance of zymotic diseases.

Internal protozoal parasites and parasitic worms breed in and are disseminated by the house-fly; so also are the fungic spores of fermentive yeasts, of moulds, and the like, but these latter are mainly disseminated by mere air currents. The eggs of tape-worms and the like are carried by dung-frequenting flies to food, especially to semi-putrid food devoured by dogs and pigs.

Some of the skin-piercing and blood-sucking flies are pestiferous in a more direct way than any of the tribes of filth and sweat-flies. They are the usual or suspected agents whereby anthrax, cattle-plague, swine fever, glanders, and other diseases are spread far and wide. Some of these last blood-sucking flies will travel with and on the bodies of transported animals for long distances; of course there can be no doubt also as to the capability of disease dissemination by the direct independent flight of flies to long distances with favouring weather and breeze. Such evils are prevalent throughout the temperate zones, but circumstances are far worse in the tropics, where Glossina morsitans, considered by some to be a near relation of our Stomoxys calcitrans, transmits the microscopic trypansomes which cause the devastating "sleeping sickness" of mid-Africa. This last reference, and other discoveries of the fly-borne germs of recurrent fevers, should bring into prominent notice a very pertinent fact; which has not yet received adequate scientific investigation. All the bites of our common blood-sucking insects, flies, gnats, midges, fleas, etc., are each kind of them wounds, sometimes very inflammatory, sometimes but little or not at all so; furthermore, the worst inflammatory wounds not uncommonly show feverish symptoms of a well marked periodic character, quiescent intervals being followed by revived inflammation in the same spot. These facts almost prove, or at least strongly suggest, an explanation that in the latter cases the source of pathogenic trouble is of a microbic character, and that periodic recurrence of feverish and inflammatory symptoms is the time of the spore-swarming of breeding microbes; the difference between these latter and those of a more severe and often fatal kind in the tropics being that the one class finds the human body a fit host in which to multiply but the other class does not, and accordingly the latter grow weaker until their breed dies out. Similar effects can be observed in cattle and other animals, but all creatures, after suffering much from fly-bites at first, afterwards become for a time more or less immune.