Fly, the popular name of the diptera, or two-winged insects, of which a familiar example is the common house fly. They have a sucking proboscis, two veined and membranous wings, and two poisers behind the wings; they undergo a complete transformation. The characters of the order have been sufficiently detailed in the article Diptera, and therefore only some of the most common flies of the family muscidoe will be noticed here. The house fly (musca domestica, Linn.) of Europe is considered distinct from the American species by Dr. Harris, who calls the latter M. harpyia; it begins to appear in houses in July, sometimes a little earlier, becomes very abundant toward the end of August, and does not disappear until killed by cold weather; the eggs are deposited in dung, in which the fleshy larvae undergo their transformations; consequently this species is most numerous in the vicinity of stables and unclean places. The swarms of summer are doubtless the progeny of a few individuals which have survived the winter in some protected nook, and are not produced from eggs laid the preceding season; it is possible that a few may pass the winter in the pupa state, and be developed by the warmth of spring.
Among the thousands of domestic flies, all are of the same size, those larger or smaller being of different species, and neither very old nor very young individuals of the M. domestica. The house fly is such a constant companion of man, that its presence in a coral or other island is sufficient evidence that human inhabitants are not or have not been far distant. Its two compound eyes contain 4,000 facets, each the cornea of a separate ocellus; the spiracles through which air enters the tracheae are provided with a kind of sieve formed by minute interlaced fibres, which prevent the introduction of dust and foreign substances. The hard parts of the proboscis are undeveloped, in their place being a fleshy tongue-like organ, or labium, bent underneath the head when at rest. Its knob-like end may be extended into two flat, broad, fan-shaped muscular leaves, by whose sucker-like surface the fly laps up liquid sweets, as sugar dissolved by its own saliva. The leaves are supported on a framework of tracheae, which end in projecting hairs, acting as a rasp on delicate surfaces, and causing a tingling on the naked skin of man. It is well known that flies, like many other insects, have the power of creeping up smooth perpendicular surfaces, and of walking on ceilings with their backs downward.
The last joint of the tarsus has two strong hooks, and a pair of membranous expansions (pulvilli), beset with numerous hairs, each having a minute disk at the extremity. There has been considerable difference of opinion as to the precise mode in which this apparatus enables the fly to walk in opposition to the force of gravity. Derham, Home, Kirby, and Spence believed that the pulvilli act as suckers, a vacuum being formed beneath, and that the insect is held up by the pressure of the atmosphere against their upper surface; others have maintained that the adhesion is due to a viscid liquid secreted from the bottom of the foot. Dr. Hooke and Mr. Blackwall assert that the soles of the feet are so closely beset with minute bristles that they cannot be brought in contact with any surface so as to produce a vacuum, and believe that the support is owing to the strictly mechanical action of these hooks. Mr. Hepworth ("Journal of Microscopical Science," vols. ii. and iii.) reconciles these apparently contradictory opinions by the conclusion that the minute disks at the end of the individual hairs act as suckers, each of them secreting a non-viscid liquid, which renders the adhesion perfect; a structure which exists on a larger scale in the feet of dytiscus and other beetles.
Mr. White, in his "Natural History of Selborne," observes, in confirmation of the views of Derham, that toward the close of the year, when flies crowd the windows in a sluggish and torpid condition, they are hardly able to lift their legs, and many are actually glued to the glass, and there die from inability to overcome the pressure of the atmosphere. It is well known that some lizards possess a similar faculty, and a similar apparatus to account for it. A dish of strong green tea, well sweetened, will be eagerly tasted by flies, and prove a certain poison; according to Mr. Spence, a netting of large meshes stretched across a window of a room lighted only on one side will not be passed by flies.-The blue-bottle or blow fly (If. [calliphora] vomitoria, Linn.) is a large, buzzing species, blue-black, with a broad, steel-blue, hairy hind body; it is found in summer about slaughter houses and all places where meats are kept, which it frequents for the purpose of depositing its eggs on animal substances.
The eggs, usually called fly blows, are hatched in two or three hours; the larvae increase so rapidly in three or four days, and are so vo-racious, that Linnreus did not greatly exaggerate when he said that the larvae of three females of this species will devour the carcass of a horse as quickly as would a lion; they pass the pupa state in the ground or in some crevice, the larval skin not being cast off, but changed into an egg-shaped case; from this they emerge as flies in a few days, or, if hatched late in the season, remain unchanged through the winter. A smaller, brilliant, blue-green fly, with black legs, much resembling the M. (lucilia) Caesar of Europe, lays its eggs on meat and the carcasses of animals.-The flesh fly (sarcophaga carnaria, Meig.), somewhat larger than the blow fly, is ovo-viviparous; it drops the living larvae on dead and decaying animal matter, and these active little scavengers commence at once their work of purification. A single female will produce about 20,000 young, which have been ascertained by Redi to increase in weight nearly 200-fold in 24 hours; Reaumur found the assemblage of embryo flies in this insect to be coiled like a watch spring, about 2 1/2 in. long when unrolled; the larva) arrive at maturity in succession, and the mother as usual dies soon after the brood is hatched.
This European species is black, with lighter stripes on the shoulders, and grayish black abdomen checkered with lighter squares. Another species of Europe is the S. mortuorum (Linn.), five or six lines long, with a golden head, grayish black thorax, steel-blue abdomen, and white wing scales. Both of these sometimes deposit their young on wounds and ill-conditioned ulcers of the living human body. The largest American species is the S. Georgina (Wiedemann), the females of which are about half an inch long; the face is silvery white, with a black spot between the copper-colored eyes; the thorax light gray, with seven black stripes; the hind body, conical and satiny, is checkered with black and white; they appear about the end of June, and continue till after the middle of August. In this genus the bristles on the antennae are plumose.-The dung fly (scatophaga stercoraria, Meig.), of a yellowish olive color, deposits its eggs in soft dung; at the upper end they have two divergent processes which prevent their sinking too far into the nidus. The S. furcata (Harris) of the United States has the same habits, and has been erroneously charged with producing the potato rot, simply because the larvae are found upon the stalks of this plant, developed from eggs laid in the surrounding manure.
The males are yellow, with hairy body and legs, and long narrow wings, and are about half as large as a honey bee; the females are smaller, less hairy, and olive-colored; both young and adult insects live upon dung, and do not injure plants.-The stable fly (stomoxys calcitrans, Meig.) is a well known tormentor of animals and man, whose skin it perforates by a painful bite in sultry weather and just before rains; it resembles very closely the house fly, except that the antennas are feathered, the proboscis very long and slender, and the size smaller; it attacks the legs, piercing through thick stockings and the thickest hair, returning to the attack as soon as driven away; it is solitary, not social like the house fly, and seldom enters houses unless driven in by bad weather; it is most abundant in August and September, when it is a great pest to horses and cattle; it is about one third of an inch long, and lays its eggs in dung, in which the young are hatched and undergo their transformations.-The cheese fly (piophila casei. Fallen.) is only 3/20 of an inch long, shining black, with transparent wings and yellowish hind legs.
By its long ovipositor it penetrates the cracks of cheese, and deposits about 250 eggs, which are developed in a few days into maggots or skippers; these larvae have two horny hooked mandibles, which they use for digging into the cheese, and for locomotion instead of feet. This larva leaps 20 or 30 times its own length, first erecting itself on the tail, then bending into a circle and seizing the skin near the tail with its hooked jaws, and finally projecting itself forward by suddenly throwing itself into a straight line. The droppings and decay caused by these larvae give a flavor to old cheese which is much relished by epicures. The wine fly, living in old casks and bottles, is also a piophila,.-There are several species of flower flies, of the genus anthomyia, of small size and feeble flight, which sport in the air in swarms like gnats, and which in the larva state are very injurious to vegetation; some of these maggots are like those of common flies, others are fringed on the sides with hair. The A. ceparum (Meig.), of an ash-gray color, with black dorsal stripes, and about half the size of the house fly, lays its eggs on the leaves of the onion close to the earth; its smooth white larvae bore into the bulb, and entirely destroy it.
The A. brassicae and A. lactucarum are equally destructive to the cabbage and lettuce; the A. raphani (Harris) attacks in the same way the radish. The A. scalaris and canicularis give rise to fringed maggots, which have been not unfrequently ejected from the human body, having probably been swallowed with vegetables in which decay had commenced; as the eggs in many instances belong to species depositing in the ordure of privies, the larvae might remain alive for a considerable period in the intestines of man; eggs of other museidce might be introduced on meats, fruits, salads, vegetables, and in impure water. In the "Transactions of the entomological society of London (vol. ii., 1837), Mr. Hope gives a tabular account of 37 cases in which maggots of the museidae infested the human body, many of which were recognized as belonging to M. domestical C. vomitoria, and S. carnaria; and many cases have since been recorded in medical journals.
House Fly (Musca domestica), magnified.
Blue-Bottle (Musca vomitoria). Larva and Pupa.
Cheese Fly (Piophila casei).
Larva of Piophila casei.
Onion Fly (Anthomyia ceparum). 1. Fly. 2. Larva. 3. Pupa.