Whereas the blue-bottle rarely enters the dwellings of mankind, except gravid females led by the sense of smell in search of fish, or flesh meat, and (less eagerly) sweets, both species of house-fly and both sexes seem to delight in the mere odour of humanity; breeding females will seek the larder and the dust-bin, but others will very provokingly pervade all quarters. Although avoiding a dark or deeply shaded room, the house-fly seems to like partial shade; it will be content to remain indoors and to rejoice in a warm kitchen, even on a hot summer's day, whilst all the other kinds of flies are enjoying the outdoor sunshine. It may be said of nearly a dozen other species, occasionally observable crawling on window panes, that they are "outdoor" flies, and that their occurrence indoors is accidental. In fact, they are mostly observed when trying to escape.

Next after human habitations, stables, cow-sheds and pig-sties are the delight of the breeding female house-fly. Round about and in these latter resorts she associates with an immense host of rather small sized flies, and amongst a few others of equal size with the skin-piercing and blood-sucking stable-fly; but many stablemen are ignorant of the difference of the two kinds of flies and of the serious suffering of their horses from the bites of the stable-fly. This lamentable ignorance was shared by the joint authors of "Humble Creatures," published in 1858, when Neo-Darwinism was in vogue, and many books were published for popularising a knowledge of common things and spreading an interest in nature-study; this publication, which is still (1914) in print and very little revised, has probably led some later would-be nature-study teachers to follow suit in confusing the characteristics of the two species. Very often the fly most numerously breeding in the manure heaps of the mews will be Borborus equinus, or some other of the same family, which are characterised by a very simple pattern of wing nervures and by the absence of squamÆ or scales behind the wings; also the ankle joints of the feet are most peculiarly short and broad. B. equinus, and a great host of other dung breeding flies of a still smaller size, may be considered beneficial insects; they do not pester cattle, and their larvÆ make food more scarce for injurious flies.

The breeding habits of the blue-bottle are very conspicuous by reason of its haste and boldness in taking possession of dead animals. It is incapable of breeding in horse or cow dung, to which latter the green-bottle fly often resorts.

The blue-bottle deposits her eggs, 500 or 600, preferably on dead fish, or flesh, and sometimes on the sores or the flesh of wounded animals, but both the house-flies preferably affect dung, carrion, garbage, and all kinds of fermenting vegetable matter. It has been commonly but not truly said that the principal breeding places of the house-fly are the mews and the farmyards where manure is allowed to accumulate; the house-fly has a preference for horse dung before cow dung, which is preferred by some other kinds of flies; however, near towns, the domestic dust-bins, heaps of market garbage, and deposits of town refuse give rise to a worse plague of house-flies than stables. All these flies deposit batches of white eggs, and are careful to place them as much as possible in crevices and shielded from exposure to strong light, or from draughts.

The two house-flies and the blue-bottle have similar larval stages, but their larvÆ, called maggots, differ. The larvÆ avoid daylight and cannot withstand dryness. As the larvÆ feed, they have the power of ejecting or excreting a juice, which dissolves the food before they imbibe the material; their mouths are suctorial and are destitute of teeth or biting jaws.

The larva of the house-fly is an eyeless and legless maggot, one half inch long when full grown and extended; twelve cylindrical segments may be counted in its body, or even thirteen if we separately distinguish the small head segment, which may be withdrawn, and but little observable; five or six rear segments are of nearly equal stoutness when only half grown; afterwards counting from the three stoutish rear segments, the others taper towards the very small head. The middle and rear segments have pad-like bristly processes underneath, which aid the maggots in creeping, in which action they also make much use of the head segment's grappling hook. The maggots feed voraciously, but they seem, like the larvÆ of the honey bee, to pass out very little anal excreta; some have thought that, like what is said of bee larvÆ, no excrement is discharged until after the imago has emerged from the puparium; but such conduct seems altogether incredible. In the bee-hive doubtless the assiduous workers ever wash their babies clean and lick up all matter, just like domestic cats and dogs, when nursing their young.

The larva of the blue-bottle, called a gentle, is proportionately larger but very similar, except that the rear segment possesses a ring of tubercles, which may have some useful function in connection with two breathing tracks, which have their orifices at that part of the body.

The larva of the lesser house-fly is very peculiar; all its segments have projecting tubercles; its whole body is rather louse-shaped, having not cylindrical but somewhat flattened segments, of which the middle are the broader, and those near the head and tail the narrower.

The transformations in the case of the blue-bottle are typical of the house-fly and others of closely related families and genera which are many-brooded within the year; these creatures develop very rapidly immediately after emerging from the egg. Some other kinds of dipterid maggots, which are single-brooded, pass a very prolonged and obscure early period of skin-shedding and non-feeding, a preparatory sort of baby-hood metamorphosis; then at last they begin to feed voraciously and to follow the general habits of other maggots. Some maggots curiously refuse to feed except in company; probably some are unable to feed on dung except where other species are providing the necessary dissolving juice.

When the common maggots or gentles have ceased feeding, they burrow into the ground or crawl away, often to a considerable distance, apparently seeking a secluded, a more wholesomely clean, and a dryer spot. During this migrating time, they are palatable food for many birds, which would not eat them in their former food-loaded or unscoured state. Indeed, it is doubtful whether either a vulture or a raven could eat a fly-blown carcase without danger of myiasic punishment. The skin of the larva whilst growing is transparent, but, when about to pupate, it thickens and becomes an opaque creamy white.

The most marvellous part of the metamorphosis of the blue-bottle is concealed, when the gentle becomes the pupa; according to RÉaumur the embryonic fly develops most curiously inside the puparium by a procedure not exactly like the change from the caterpillar to the chrysalid in the case of the butterfly. After a pause of a day or two, the front segments of the fully fed maggot contract, so that the body assumes a barrel-like shape; the skin then hardens, and turning a reddish brown it becomes a much contracted shell or case called the puparium. However, the long slender maggot has done something more than merely shrink and shape itself conformably to the case; it has withdrawn its embryonic head, so small as to be hardly distinguishable microscopically, together with its embryonic legs, wings and thorax into its embryonic abdomen! As the development proceeds, and the embryonic members of the future perfect insect acquire their destined shape, the immensely increased head and the thorax with its appendage members slowly emerge, and the partly inverted integument of the abdomen rolls back, disclosing the shape of a fly not before recognisable.

Other naturalists would have it believed that the true account of the transformation is as follows,—when the maggot has shrunk and freed its body inside its skin which forms the case or puparium, all its pre-existing internal organs become absolutely dissolved; then out of the fluid mass a new growth ensues, constituting the pupa with its recognised shape. This account is the one represented in most modern entomological books, and is based partly upon B. T. Lowne's monographic work on the blow-fly.

The comparative embryologist of our day is inclined to be a hyper-theorist, and so it seems that some have not remained content with either of the above accounts; to them, apparently, the production of the large and complex head of the imago out of a single small anterior segment of the maggot requires a more recondite explanation, and must be brought into harmony with analogous facts. To this end some degree of linked support is found by the investigations of microscopic anatomy, and it has been conjectured that not one or two head segments, but five are lying blended and embryonically hidden in the larvÆ, all ready to bud forth. However, for fear of wearying too much with the theories of advanced erudite scientists, the following jeu d'esprit is presented, instead of a more elaborate and sober attempt, to lure the unscientific lay reader to an extreme hypothetical conception of the "essential unity" underlying the apparent diversities of Nature within that vast domain of the Kingdom of Fauna, which is obviously outside the later creation of a vertebrate Animalia.

The futurist's dogmatic CREDO of creative progress, "For him who would meritoriously pass his histological examinations, and qualify as a Professor and Doctor of Science, above all it is necessary that he should acknowledge the unicellularity of the primÆval OVUM (or egg), whence proceeds the seventeen-segmented boneless ANNELID (or worm), out of which there develops the quadrangular articulated crustacean INSTAR (or shell-encased aurelian), which metamorphosises into the winged IMAGO (the angelic? or diabolic? fly); in the contemplation of this knowledge alone is there supreme Darwinian Modernismal salvation and felicity." Amen.

In view of the prosaic illustration of transmutation, figure 5 above, the futurist disciple will have to accept the seventeenness of segmentation by something like faith without sight.

The quadrangularity of the crustacean stage is based upon the idea that the wings bud out from the two upper corners, whilst the legs develop from the lower corners of the transmuting instar.

Perchance the reader will desire information about the use of this curious word "instar," which has not the honour of notice in Dr. Sir J. Murray's New English Dictionary. One might well feel proud of the opportunity of adding the smallest item to such a stupendous and monumental work, but I fear I am only qualified to venture a fair guess. Virgil, I believe, used this term in allusion to the legendary wooden horse of the Greeks at their siege of Troy. Some time less than one hundred years ago entomologists recognised that the words aurelian, chrysalis, and pupa were none of them an inherently fit term of general application to the stage of insect life to be indicated. After many attempts, this latest proposed substitute seems to be gaining favour.

The fly emerges after bursting apart the first four segments of the puparium; this it does by a curious provision, whereby it can inflate a chamber in its head in a queer, balloon-like fashion, making a bag-like extrusion, which it uses as a punching and pushing machine.

After emerging from the ground, the fly withdraws the bag-like extrusion and cleans itself. Its body soon grows fit and it becomes very active, as long as daylight and warm weather favour it; otherwise it seeks shelter and becomes quiescent; however, artificial light and heat will awaken it to nocturnal activity. Sweets, carrion, and filth are all attractive to the blue-bottle, but the house-fly and the lesser house-fly also find extraordinary attraction in both man and his dwelling.

Considering the superfluity of other flies, and the multitude of other insects ever ready to do duty as devourers of carrion, garbage, and filth, the scavenging services of the larvÆ of the house-fly can be well dispensed with.

In civilised communities cremation in a refuse destructor is the only sanitary method of treating town refuse. The economic value of the fly is very little, and consists merely in its food value for certain birds.

In warm weather the scavenging capabilities of all the carrion and filth feeding maggots are very remarkable, and there appears no exaggeration in the statement by LinnÆus, that the progeny of three flesh flies can eat up the carcase of a horse sooner than it could be devoured by a lion.

When a batch of eggs has matured in the abdomen of the female, she is most careful in the location and manner of their disposal. Guided by the sense of smell, she will not lay her eggs except in contact with food, or in places securing her progeny access to their intended food. By the use of her soft, slender ovipositor, which is telescopically extensile and flexible, the eggs are deposited in shaded and concealed situations.

The house-fly is credited with laying batches of eggs at intervals, perhaps four or more times, and about 150 on the first occasion, then 100, and less on subsequent occasions. Under favourable circumstances the eggs may hatch within a few hours of their being laid. The maggots of midsummer broods may be full grown and pupate in six days, and the perfect insect may emerge from the puparium in another ten days of warm weather, but in cold weather the pupÆ of autumnal broods may remain dormant for several weeks, or even months. When nine or ten days old the mature fly may begin to lay eggs; hence, with such a life-cycle, in a month of very favourable weather the progeny of a single pair may number, say, 500; in two months' time the number may become 250 times 500; and in three months' time many millions!