FLIES
PART I
THE HOUSE-FLY (Musca domestica)
Musca est meus pater, nil potest clam illum haberi;
Nec sacrum nec tam profanum quidquam est, quin Ibi ilico adsit.
(PLAUTUS, Mercator.)
‘THE common house-fly [says Ruskin] is the most perfectly free and republican of creatures. There is no courtesy in him; he does not care whether it is a king or clown whom he teases, and in every step of his swift mechanical march and in every pause of his resolute observation there is one and the same perfect expression of perfect egotism, perfect independence and self-confidence and conviction of the world having been made for flies. Your fly free in the air, free in the chamber, a black incarnation of caprice, wandering, investigating, fleeting, flitting, feasting at his will with rich variety of feast from the heaped sweets in the grocer’s window to those of the butcher’s back yard, and from the galled place on your horse’s neck to the brown spot on the road from which, as the hoof disturbs him, he rises with angry republican buzz; what freedom is like his?’
The house-fly is all that Ruskin describes it to be, but it is more. It is the most cosmopolitan of insects. Wherever man is there is the fly. It is found—
From Greenland’s icy mountains
To India’s coral strand.
But it is naturally more frequent in warm climates than in cold, as the rate of its development depends very largely upon an average high temperature.
Unlike the lice and the bed-bug, the fly like the flea, passes through a complete metamorphosis—egg, larva, pupa, and imago. It will breed in almost any rotten matter, whether vegetable or animal, and it breeds most successfully, as Gordon Hewitt has pointed out, when certain processes of organic fermentation are taking place in its breeding-place. Probably the fermentation has a favourable effect upon the food of the larvae. Undoubtedly the place most readily selected by the female for laying her eggs is stable-manure. A few years ago there was a remarkable reduction in the number of house-flies in London, and Lord Montagu of Beaulieu attributed this reduction to the refreshing and insecticidal petrol vapour with which the streets of that town were then bathed.
FIG. 14.—Mass of eggs of M. domestica. (From Gordon Hewitt.)
I do not know what experiments Lord Montagu had made on the subject of the insecticidal value of petrol vapour, but the ordinary man in the street attributed—and I think more correctly—the diminution of the plague of flies to the absence of the nidus in which the female fly lays her eggs. Stable-yards had been turned into garages. But flies will, indeed, breed in almost any kind of dejecta—including the human—and in rotten straw, rotten wool, cotton garments, decaying vegetables and fruits, bad meat, rotten grain, and even in spittoons, but they prefer horse-manure.
FIG. 15.—Eggs of M. domestica, × 40. (From Gordon Hewitt).
In our country house-flies usually begin to breed in June and July, continuing well on into October if the weather be but warm. Their greatest activity is, however, in the hotter month of August and the beginning of September. But in warm stables, restaurants, and kitchens flies are able to reproduce the whole year round. A single fly will deposit at one time 100 to 150 eggs, and in the course of her summer life may produce five, or even six, batches of ova of this size. The eggs are pearly white, elongated structures, with two converging lines, along which the egg-case will ultimately split to give exit to the larva. The eggs are laid, by means of a long ovipositor, a little way beneath the surface of the dung-heap in a position where they will not readily be dried up. In favourable conditions the eggs hatch in from eight to twenty-four hours.
The first larva is legless, tapering towards the head, which bears a pair of breathing-holes, or spiracles; the body is much stouter towards the hinder end. On the whole it is a white, unpleasant-looking maggot, called by freshwater-fishermen a ‘gentle.’ By contracting and expanding its body it pushes its way through the moist, semi-liquid surroundings. The skin is usually moulted some twenty-four hours after birth, but all these time-limits depend much upon the temperature and favourable conditions. With normally high temperatures—say, with 30° C. to 35° C.—the larva will become fully grown in five or six days. The third and final larval stage, after the second moult or ecdysis, lasts three days, and when fully grown the maggot is now about half an inch in length. Externally, twelve segments are visible, but the internal anatomy shows that thirteen are really present, though one is almost ‘masked.’
FIG. 16.—Abdomen of female house-fly, showing the extended ovipositor. (From Gordon Hewitt.)
It is only during these larval stages that the insect grows, and it is never more bulky than in the third larval stage. Now it leaves the moist situation, in which it has flourished, and, crawling through the manure, seeks some dry or sheltered corner. For a time it rests, and then after an hour or two’s quiescence it retracts its anterior end and assumes a barrel-shaped outline, its creamy white colour slowly changing to a mahogany brown. The larval skin forms the pupa-case, and within this pupa-case the body of the larva undergoes a wonderful change, far greater than even human beings undergo at the time of puberty. Many of its organs are disintegrated and re-formed, and in the course of three or four days the white, legless, repellent maggot, who ‘loves darkness rather than light,’ is changed into a lively, flying insect, seeking ‘a place in the sun’ and the companionship of man. As the Frenchman said of the pig which goes into one end of the machine in the Chicago meat-factory as live pig and comes out at the other end in the form of sausages, ‘Il est diablement changé en route.’
FIG. 17.—Mature larva of M. domestica. a.sp, Anterior spiracular process; an.l, anal lobe; sp, spiniferous pad. I-XIII, Body segments. (From Gordon Hewitt.)
In a very short time after leaving the pupa-case the adult fly has stretched her wings, the chitin of her body has hardened, and she flies away ‘on her several occasions.’
FIG. 18.—‘Nymph’ of M. domestica dissected out of pupal-case about thirty hours after pupation. an, Swellings of nymphal sheath marking bases of antennae; cx, coxa of leg; lb, labial portion of proboscis sheath; lbr, labral portion of same; n.sp, spiracular process of nymph; w, wing in nymphal alar sheath. (From Gordon Hewitt.)
FIG. 19.—Pupal-case or puparium of M. domestica from which the imago has emerged, thus lifting off the anterior end or ‘cap’ of the pupa; ventro-lateral aspect. a.sp, Remains of the anterior spiracular process of larva; l.tr, remains of the larval lateral tracheal trunk; n.sp, temporary spiracular process of nymph; p.sp, remains of the posterior spiracles of larva. (From Gordon Hewitt.)
Flies become sexually mature in a week or ten days after emerging from the chrysalis-case, and are capable of depositing their eggs four days after mating, so that if the conditions be indeed favourable the whole development from the egg to the perfect fly may be accomplished in nine or ten days, and the second generations are able to lay their eggs ten days later. The appalling fecundity of such an insect explains the fact that in the hotter parts of the world nearly every edible thing seems to be covered with them.
The proboscis of a fly can only suck up liquid food; and when we see it feeding on solid substances, such as sugar, it has really dissolved the sugar by depositing some saliva on it, and is sucking up the sugary solution so produced. It not infrequently regurgitates its food in a spherical drop, which it generally re-absorbs.
As we have seen, flies are very susceptible to temperature, and with the approach of cold weather they seem to die. We used to think that some, in a state suspended animation, ‘carried on’ through the winter months. This is, however, ‘non-proven.’ Many of them undoubtedly die in the autumn, as bees die, of old age. They are literally worn out. But a great number fall victims to a parasitic fungus called Empusa. Flies killed by this fungus are frequently to be seen in autumn, hanging dead on windows, &c., surrounded by a little whitish powdery ring of spores formed by the fungus.
Flies, like many other insects, are extremely difficult to keep alive in captivity, and few have succeeded in rearing them for more than a month or two. At one time, as we have said, it was thought that those flies which survive the winter were fertilised females of the younger broods, and that during the winter they subsisted on their ‘fat bodies.’
FIG. 20.—M. domestica in the act of regurgitating food. × 4½. (From Gordon Hewitt.)
Doubt has recently been thrown on this theory, and in a recent report[8] of the Local Government Board Dr. Newsholme sets forth the results of the researches of Dr. Monckton Copeman and Mr. E. E. Austen in the following words:—
Until recently there was general agreement that a certain number of flies managed to survive the winter and spring by hibernating in dark nooks and crannies in dwelling-houses, or, as contended by Dr. Laver,[9] in various sheltered situations outside dwellings—such as the under-surface of the thatch of farmyard stacks. The researches of Mr. Jepson and others have shown that, during the period extending from late autumn to early summer, flies may be found occasionally in all active conditions in warmed houses, and especially in such places as kitchens and bake-houses, where the temperature is kept relatively high; and further, that under these conditions, and in presence of sufficient food material they may even continue to breed. Doubt has, however, been expressed as to whether a sufficient number of flies remain in active condition in these localities to perpetuate the species and to start the rapidly multiplying generations of the following summer. As to whether flies can persist through the winter in other than adult form practically nothing is known.
In view of the importance of obtaining further information on these points, some inquiries were undertaken into the hibernation of flies, the results of which were set out in a communication by Dr. Copeman published in the sixth report of this series. Arrangements were made with a working naturalist for the collection of any flies that could be found in situations like those which Dr. Laver and other observers had found to be favourite winter quarters of hibernating flies. In view of the need, pointed out by Howard, for expert identification of the species of all flies captured in a dormant condition during the winter months, the co-operation of Mr. Austen of the British Museum (Natural History) was obtained, and to him all the flies collected were submitted for examination. The one specially interesting and unexpected point emerging from this inquiry was that not a single specimen of the house-fly (Musca domestica) was met with among the considerable number of hibernating flies caught in situations which have hitherto been regarded as the special habit of this fly. Under these circumstances it was felt that further detailed investigation of the matter was needed; and, accordingly, inquiry on a more extended scale, and covering—as it proved—an extensive area, was initiated and carried through during the past winter.
* * * * *
Once more, the results obtained afford no support to the belief that house-flies hibernate, in this country, in the adult state; and the problem as to the manner in which the interval between one fly-season and the next is bridged over still remains unsolved.
Gordon Hewitt, Copeman, Howlett, Merriman,[10] and others, have made experiments as to how far a fly can travel. Marked flies have been taken within forty-eight hours at distances ranging from 300 yards to a mile. Apparently the direction of the wind plays a considerable part in the distance they travel.
The importance of the house-fly as a carrier of disease, especially bacterial disease, has recently been recognised especially in times of war. Moses was as great as a Principal Medical Officer as he was as a Director of Supplies; and this is shown in Deuteronomy, chapter xxiii, where he deals with the need of strict hygiene in the camp.
In the middle of the last century already attention was being drawn to the fact that the house-fly and the blow-fly transmitted various diseases. But it was during the Spanish-American War and the South African War which followed shortly afterwards that the part played by these pests in conveying enteric became definitely established. Flies coming straight from the latrines, with their legs and their wings and their proboscides soiled with typhoid bacilli, would enter the camp and the tents of the soldiers and settle on their food-supplies—crawling over their jam, floating in their milk. Thirty per cent. of the deaths in our South African War were due to typhoid fever. The bacillus, as is well known, is capable of existing for a long time and of persisting alive in the alimentary canal of the insect. Dr. Graham-Smith has shown that the bacilli may remain active for six days after feeding, and that the feet of flies which have the bacillus on them are capable of infecting surfaces upon which they walk for at least two days after first coming in contact with the germs that cause ‘enteric.’
FIG. 21.—A, Foot of a fly, showing hairs bearing bacteria; B, a single hair more highly magnified; C and C´, bacteria. Diagrammatic.
Faichne reared maggots in dejecta infected with typhoid bacilli, and he was able to show that the flies into which these maggots turned contained virulent typhoid germs in their intestines. There is absolutely no doubt that typhoid is largely conveyed by the agency of these insects; and as flies are perfectly controllable, if ‘the people will but have it so,’ it is one of the disgraces of our civilisation that this disease should be so prevalent.
The protective inoculation against enteric is now almost perfect, and its value is shown by quotations from a leaflet issued by the Research Defence Society:—
Sir William Leishman, in a letter published during the present war, August 22, 1914, says: ‘The benefits of inoculation are so well recognised in the regular forces that we find little difficulty, in foreign stations, in securing volunteers for inoculation: for instance, about 93 per cent. of the British garrison of India have been protected by inoculation; and typhoid fever, which used to cost us from 300 to 600 deaths annually, was last year responsible for less than 20 deaths. Inoculation was made compulsory in the American army in 1911, and has practically abolished the disease; in 1913 there were only 3 cases, and no deaths in the entire army of over 90,000 men.’
In Avignon, in the south of France, during the summer of 1912, typhoid fever broke out in the barracks. Of 2053 men, 1366 were protected and 687 were not. The non-protected had 155 cases of typhoid, of whom 21 died; the protected had not one case. In the winter of 1913 the French Senate resolved that the protective treatment should be made compulsory throughout the French army; and, in special circumstances, among the reservists.
FIG. 22.—Chart illustrating the relation of the numerical abundance of house-flies to summer diarrhoea in the city of Manchester in 1904. Prepared from statistics and chart given by Niven. (From Gordon Hewitt.)
Infantile diarrhoea, which so afflicts the crowded, poorer quarters of our cities in the summer, is another disease intimately associated with Musca domestica. But that is hardly a disease likely to trouble the soldiers. The tubercle bacillus is another germ conveyed by flies. House-flies are particularly fond of feeding on saliva; and Hayward, Lord, and Graham-Smith have obtained virulent bacilli from the intestines and dejecta of flies which had been fed on tubes containing tuberculous sputum. These experiments have been amply confirmed by other workers. Anyone who has ever been in Egypt will remember the terrible sight of the flies attacking little children suffering from ophthalmia and it is believed that the wide prevalence of this most pitiful trouble is attributable to the abundance of flies—the flies of Egypt, a plague even in the times of the Pharaohs. Things do not alter much in Egypt, and the Biblical plagues are wont to recur.
Another disease—anthrax, or wool-sorter’s disease—may be conveyed by the same carriers from infected cattle to man, and there is a good deal of epidemiological and bacteriological evidence available to show that flies play an important part in the spread of cholera, which is now threatening the soldiers in the eastern seat of the war, and possibly in disseminating the organisms which cause yaws and tropical sore.
It will be noticed that the fly is not a necessary second host for any of these germs. They are conveyed, as if by an inoculating needle, by contact with the proboscis or the legs or some other tainted organ of the fly. The bacilli, however, pass through the alimentary canal apparently unchanged and unharmed, and are deposited either with the regurgitated food from the fly’s stomach (Fig. 20), or with the dejecta of the insect. There is no subcutaneous inoculation—such as takes place in the case of the mosquito when it conveys malaria, or in the case of the tsetse-fly when it conveys sleeping sickness—where the disease-causing organism is injected into the human body. The action of the fly is mechanical, but none the less efficient. The poisoning of the soldiers’ food-supply is its chief rôle in war.