It is one of those facts which not unfrequently occur in science that we know less about the life-history and habits of the commonest insects than we know about scarce and remote species. For instance, the life-history of the common house-fly, one of the most widely distributed insects in the world, is as yet very incompletely known.

It was LinnÆus who first described this insect and named it Musca domestica, and de Geer who, in the middle of the eighteenth century, first described its transformation. In 1834 BouchÉ described the larva of the insect as living in the dung of horses and fowls. In 1873 the well-known American entomologist, A. S. Packard, reinvestigated the question, and L. O. Howard has recently written on the subject. In our own country C. Gordon Hewitt is publishing a monograph on the house-fly, which will, when completed, fill a long-felt want. Packard noted that in the August of 1873 the house-fly was particularly abundant, especially in the neighbourhood of stables. He was able to observe the insects laying their ova in clumps containing some 120 eggs in the crevices of stable manure, ‘working their way down mostly out of sight.’ The eggs hatched in about twenty-four hours, but he noticed that those hatched in confinement required from five to ten hours longer, and that these larvÆ when hatched were smaller than those hatched out in the open. The eggs are oval and cylindrical, one twenty-fifth to one-twentieth of an inch long and about one-hundredth of an inch wide, and of a dull, chalky-white colour.

The little larva has not been seen emerging from the egg-case, but probably, as in the case of the meat- or blow-fly, Musca vomitoria, the eggshell splits longitudinally and the maggot pushes its way out. The length of the newly-hatched larva in its first stage (or instar) is seven-hundredths of an inch, and it remains in this stage about twenty-four hours, when it casts its skin and appears as a larger maggot three-twentieths of an inch long. In this condition it remains from twenty-four to thirty-six hours. After a second moult the maggot attains the length of one-quarter of an inch, and in this stage it remains five or six days. During its life the larva moves actively about amongst its surroundings, eating up the decaying matter, but avoiding bits of straw and hay. There is some evidence to believe that, if pressed for food, larvÆ may devour one another. After living altogether some five to seven days, the larva somewhat suddenly turns into a dark brown pupa or chrysalis. The transition takes place very rapidly—in the course of a few minutes—and the pupa remains enclosed in the last larval skin. After another period of five to seven days in normal circumstances the insect hatches out, at first running around with soft and baggy wings, which, however, soon stretch out, harden, and dry. It is worthy of note that whereas Howard found the complete metamorphosis to take ten days, and Packard from ten to fourteen days, in the cooler climate of Manchester Hewitt finds it takes from twenty to thirty days. The last named gives some interesting particulars as to the effect of the weather upon the rate of development. It is believed that many flies pass the winter in the pupa state; the adult fly also survives the cold weather hidden away in cracks and crevices, from which it may from time to time emerge when the sun shines warmly.

When the larvÆ are reared in too dry manure, they attain only one-half their usual size. Too direct warmth and the absence of moisture and available semi-liquid food also tend to dwarf them.

A word may be said about the distribution of the insect. It is practically cosmopolitan. As Mr. Austen records:

It is carried all over the world in ships and trains, and seems to be equally at home in the high latitudes of Finmark or in the humid heat of Equatorial Brazil.

The diseases which flies convey from man to man—which rendered them by no means the least formidable of the plagues of Egypt, and fully justified Beelzebub’s title of the ‘Lord of Flies’—are for the most part conveyed mechanically. The proboscis acts as an inoculatory needle. No part of the life-history of the disease-causing organism must necessarily be carried on in the body of the fly; it is conveyed mechanically and without change from an infected to a healthy subject. The mouth parts can pick up the anthrax bacillus, and if the fly then alight upon a wounded surface it will set up woolsorter’s disease. It, together with the flea, is accused of transmitting the plague bacillus, not only from man to man, but from rat to man. Flies are active agents in disseminating cholera; and anyone who has watched them clustering around the inflamed eyes of the children in Egypt, or in Florida, will not readily acquit them of being the active agents in the spread of inflammatory ophthalmia or of ‘sore eye.’

It is worthy of note that after exhaustive experiments on the tsetse fly (Glossina palpalis), which conveys that most fatal of diseases, sleeping-sickness, Professor Minchin and his colleagues, Mr. Gray and Mr. Tulloch, have come to the conclusion that the Protozoon (Trypanosoma gambiense) which causes the disease does not—as might be expected—pass through certain stages of its life-history in the fly, but is mechanically conveyed upon the biting mouth parts of the insect. The deadly parasite is, indeed, so easily cleaned off these appendages that a single bite is sufficient to wipe them off. A tsetse fly which has bitten an infected person will set up the disease in the next person (or monkey) it bites; but the insertion of the proboscis, quick and instantaneous as it is, serves to clean it—to wipe off adhering trypanosomes, and if it now bite a second person (or monkey), it fails to convey the disease. This is a most important discovery, and contrary to what we should have expected; but our knowledge of the history of the genus Trypanosoma is still too small to justify generalization, difficult as it is to avoid it. The diseases which in our country are disseminated by flies are all bacterial and all mechanically conveyed.

In passing, it is worth recording that, contrary to the usual statement that tsetse flies are confined to the continent of Africa, Captain R. M. Carter[8] has recently brought some back from the Tabau River and from other localities in South Arabia. Mr. Newstead has recognized the specimens as belonging to the species Glossina tachinoides. It evidently does not live on big game here, since, except the gazelle, game is absent. The Bedouins say that it bites donkeys, horses, dogs, and man, but not camels or sheep. It is at times so troublesome as to force the natives to shift their camps.

The common house-fly has been known for some time to be an active agent in the dissemination of bacterial diseases. In intestinal disorders—such as cholera and enteric fevers, which are caused by micro-organisms, the flies convey the bacteria from the dejecta of the sick to the food of the healthy. In the recent war in South Africa they are described in the standing camps as dividing their activities ‘between the latrines and the men’s mess-tins and jam rations.’[9] In the Spanish-American War in Cuba, and in the South African War, and in several recent outbreaks of enteric fever in the British army in India, flies have been proved to be the carriers of the Bacillus typhosus. Dr. Veeder[10] writes:

Similar records come from the Boer camp at Diyatalawa in Ceylon. The bacilli are conveyed direct, just as they might be by an inoculating needle. They do not pass into the body of the fly, neither do they undergo any part of their life-history in its tissue.

Dr. Sandilands[11] has recently investigated outbreaks of epidemic diarrhoea. He points out that the prevalence of diarrhoea follows the earth’s temperature, and does not follow the temperature of the atmosphere. It is a well-known fact that this illness is more prevalent in the houses of the poor than in the mansions of the rich. As Dr. Newsholme, late Medical Officer of Health for Brighton, said:

Compared with cow’s milk, which nourishes a very numerous progeny of bacteria, the bacterial content of NestlÉ’s milk is very low, according to Dr. Sandilands. In certain seasons the cow’s milk is exposed to temperatures which favour an enormous multiplication of bacteria, and yet it is not then a frequent source of diarrhoea—in fact, mere numbers have little or no influence on the incidence of the illness. The greater number of cases are due to infection conveyed from some patient in the near neighbourhood and conveyed mechanically by flies.

The great attraction of the sweetened condensed milk for flies to some extent explains the greater prevalence of infantile diarrhoea among children fed on this preparation.

As was stated above, one of the most remarkable features in the prevalence of infantile diarrhoea is that it follows the rise and fall of the earth’s temperature, and not that of the air. In the same way the number of house-flies does not reach its maximum with the first burst of hot weather. The prevalence of these insects follows rather than coincides with periods of great heat. The flies, in fact, lag behind the air temperature and persist for a time after the hot weather has ceased. In other words, the meteorological conditions associated with an increase or a diminution of the prevalence of diarrhoea exercise a similar influence on the prevalence of flies.

The transference of the Filaria bancrofti, whose presence in the human body in the adult stage is associated with various diseases of the lymphatics, the most pronounced of which is the terrible elephantiasis, is due to more than one species of gnat or mosquito. It is true that no one has ever seen the actual transference of the Filaria from the biting organs of the Culex, Anopheles, Panoplites, or Stegomyia into the human body, but the circumstantial evidence is so strong that on it any jury would convict. NoÈ and Grassi have demonstrated a similar mode of infection for the Filaria immitis, which exists in the adult stage in such incredible numbers in the cavity of the right side of the heart of dogs, especially in tropical and in sub-tropical countries, that it is difficult to see how the circulation can be maintained at all. It is therefore interesting to note that the proboscis of our common house-fly frequently harbours a larval nematode which has been described by Carter[12] under the name of Habronema muscÆ; and again (if it be the same species) by Generali[13] under the name Nematodum sp. (?), and again by Piana,[14] who is inclined to think it is the larval form of Dispharagus nasutus (Rud.). What the further history of this parasite is we do not conclusively know, but, judging by analogy—and in the case of the grosser parasites it is not always wise to do that—the nematode probably develops in some higher animal which eats the fly. Piana brings forward a good deal of evidence that this is the domestic fowl.

Another parasite which attacks flies is the fungus or mould Empusa muscÆ, whose growth is fatal to the insect. The hyphÆ penetrate into the body, and as they grow weaken the fly until it is unable to lift a leg, but remains glued by its viscid feet to the object upon which it rests. The fungus spreads and radiates out in all directions, covering the fly as with a velvety pile, and giving off countless minute spores, which are blown away, to alight, if they are lucky, on a further victim.

I think enough has been said to prove that flies are a very real danger to our community. I have refrained from giving the appalling statistics of our infant mortality, partly because of the difficulty of discriminating between the claims of the flies and those of other agencies which affect the lives of our babies—e.g., the insurance companies which do a large trade in insuring infants. Legislation has not attempted to control the latter. Sanitation might do much to destroy the former. In well-administered towns slaughterhouses no longer ‘fill our butchers’ shops with large blue flies’; they have been replaced by abattoirs, under proper inspection. Stables should also be segregated or controlled. The practice of backing the mansions of Berkeley Square by stable yards should either be given up, or the manure-heaps in which the flies breed should be under cover so close as to prevent the access of the fly. A layer of lime spread over the manure effectively prevents the fly laying. Creolin, in its cheap commercial form, is also recommended, sprayed over the manure-heaps every two or three days. It not only deters flies from ovipositing, but should they succeed in doing so it kills the resulting larvÆ.[15]

Ross has shown us how to clear Ismailia of malaria; the Americans have rid Havana, for the first time in a century, of yellow fever; the same could be done with flies, if only the people liked to have it so. The motorcar, with all its destruction of nervous tissue, its prevention of sleep, its danger to life and to limb, has one great merit—it affords no nidus for flies.