HOUSE-FLY. Although extremely abundant in individual representatives, by habit specially attached to mankind, of widely extended range (North American and Abyssinian exponents are absolutely identical with European), familiar from the earliest times, engrafted upon the literature of all nations by proverbial and poetical allusions, and of later years affording material for much scientific microscopical investigation, the dipterous species known to naturalists as Musca domestica apparently has two pecu liarities opposed to these premises: one, that its lack of salient external features would puzzle any but a profound dipterologist to define its specific attributes with absolute certainty; the other, that its earlier life history and trans formations remained practically unknown (at all events to orditiary readers) up to the year 1873. It is scarcely within the province of this work to diagnose species; the instincts of the majority of readers will probably direct them at once to the right insect, which may be roughly described as a quarter of an inch long, black, hairy, with a reddish oval vertical spot and golden orbits, three grey longitudinal bands on the thorax, an interrupted yellowish band at the base of the abdomen, which has lateral and apical golden spots, and the base of the wings yellowish-white. But not only allied species are liable to be confused with the house-fly; representatives of other genera, such as Anthomyia, Tachina, and Stomoxys, are often mistaken for it, and a puncture from the sharp beak of the latter fly has often caused a wrong charge of blood-sucking to be brought against the subject of this notice, which has a short fleshy bilobed tongue incapable of penetrating the skin, though provided with a terminal framework of tracheal tubes, acting like a rasp, by using which it often annoys us in the heat of summer. As regards the second point, Linnueus, who first named the fly, left its transformations undescribed; De Geer in 1776, and Bouch in 1834, described the larva and pupa, and correctly defined their habitat; but it has been reserved for Dr A. S. Packard, jun., the well-known American entomologist and biologist, to make a thorough investigation of their whole economy, which he has published in the Proceedings of the Boston Society of Natural History, vol. xvi. (for 1873-74), pp. 136-150, pi. iii.
The minute dull chalky white eggs (usually about 120 in number), elongate oval and cylindrical in shape, are laid by the parent fly in crevices of fresh manure in or about stables, heat, and especially moisture, being required for their development. The larvae are hatched in twenty-four hours, and pass through three stages, averag ing from five to seven days in all; in the second of its stages, the larva has been observed to increase by one-third of its length in twenty-four hours. They resemble those of the well-known meat fly, Callipliora vomitoria, but are smaller, longer, more slender, trans parent, smooth, and shining, and regularly conical. The prop-leg at the apex is also much smaller, and cannot be seen from above when the larva is in motion. They eat the decaying parts of the manure, leaving the bits of hay and straw. The puparium, or pupa-case, is a quarter of an inch long, cylindrical, and dark brown, closely resembling that of Stomoxys calcitrans, from which it chiefly differs in the larger and squarer anal spiracles and the smoother apex. The enclosed pupa is of the usual type of the cyclorhaphous Diptcra, and is readily distinguishable from that of Stomoxijs by its broad spatulate labium and curved maxillary palpi; it rests in the case with the hard framework of the jaws of the old larva skin next the ventral side; and when the fly pushes its way out, after remaining from five to seven days as a pupa, the upper end of the case splits off just behind the suture between the thorax and abdomen. The term "pupa" is here used in a general sense, since intermediate stages of de velopment (variously called "pseudo-nymph" or "semi-pupa") in that condition occur in the Muscidce, as in Hymcnoptcra, Colcoptcra, &c.
On leaving the pupa-case, the fly runs about with its wings soft, small, and baggy, pressed to the side of the body, much as in the pupa. It is pale, with the colours not set, and the mem branous portion of its forehead constantly distends with air as the fly moves, being connected with the trachea?. From Mr Lowne s observations on the anatomy of the blow-fly, this organ is evi dently employed for pushing away the end of the puparium when the pupa slips out of its case.
The whole period of evolution being thus from ten to fourteen days only, and the number of eggs laid by each female fly so numer ous, it will be readily seen that any slight personal inconvenience to man, as produced by the habits of the perfect insect, are much more than compensated for by the unceasing labours of its larvae as scavengers; the benefit being the more direct as the work is invariably done close to human habitations. The workings of the law of nature, by which an excess of increase in any one species is checked, are conspicuously shown in the case of this insect. Not only do the ordinary parasites of its own class (some Hymenopterous, and in one recorded instance Coleopterous) attack it in its earlier stages, but certain common birds are particularly addicted to it in the perfect state (in which also a G /ielifer, a minute European representative of the scorpions, has also been found parasitically attached to it). The vegetable world also supplies some lethal agents in the shape of fungi (notably Empusa muscce), individuals destroyed by which are constantly to be seen in autumn unable to move, and distended or ruptured by the expansion of the internal growth, the white spores of which are finally to be observed scattered round their victim.
Trivial as the house-fly may appear even to entomologists, it is to be noted that recent observations by the German biologist Weissman on its development have resulted in his discovery of its possessing "imaginal discs" in the early larval state a structure deemed of sufficient value to suggest a new division of the whole Insecta into "Discota" and "Adiscota."
