HOUSE-FLY. Although extremely abundant in indi vidual representatives, by habit specially attached to mankind, of widely extended range (North American and Abysof later years affording material for much scientific microliarities 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 transformations remained practically unknown (at all events to ordinary 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 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, Linnaeus, who first named the fly, left its transformations undescribcd ; De Geer in 1776, and Boucho 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 ilIatural History, vol. xvi. (for 1873-74), pp. 136-150, pl. 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, averaging 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, Calliphora vondloria, but are smaller, longer, more slender, transparent, 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 puparittm, 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 eyelorhaphous Diplcra, and is readily distinguishable from that of Stontoxys 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 lly 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 development (variously called " pseudo-nymph " or " semi-pupa ") in that condition occur in the Muscidai, as in Hymenoptera, Colcoptcm., &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 membranous portion of its forehead constantly distends with air as the fly moves, being connected with the tracheae. 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 puparimn 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 numerous, 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 larva: as scavengers ; the benefit being the snore 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 iu its earlier stages, but certain common birds are particularly addicted to it in the perfect state (in which also a Chelifer, 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 muscw), 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."
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