DIPTERA (Gr. two-winged), an order of insects, which received from Aristotle the name it still bears. Its distinguishing characters are so obvious that it has been acknowledged, with little change of its limits, by almost all naturalists. FLY is a popular name very generally applied to dipterous insects, and often with some distin guishing prefix (as house-fly, flesh-fly, blow-fly, bat-fly, crane-fly, etc.), although it is sometimes used with such prefix to designate insects not belonging to this order (dragon-fly. (lay-fly, May-fly, etc.). Midges, gnats, and mosquitoes are also dipterous insects. In the number of species which it contains, this is one of the most extensive orders of insects: some of the species are also remarkable for the immense multitudes in which they appear; and although most of them are of small size,.and few attract us by brilliant hues, not a few are important on account of the annoyance or mischief which they cause, either in their perfect or in their larva state; whilst many of their larva (maggots) are also very useful in consuming putrescent animal matter, which might otherwise prove a source of pestilence.
The D. have only two wings, which are membranous and simply veined. A little behind the wings are two small slender organs called halteres, poisers, or balancers, the use of which is not well known. They are usually present even in those exceptional insects of this order in which the wings are not developed. The head of the D. is gen erally in great part occupied by the large compound eyes, which often contain thou sands of facettes; and besides these, three simple or stemmatic eyes (ocelli) are often also present, placed upon the crown of the head. The mouth is formed exclusively for suction, and is usually furnished with a short membranous suctorial proboscis, com posed of parts which represent, although so differently modified, the portions of the mouth in coleopterous and other masticating insects, some of the parts, however, often disappearing. The proboscis of many is capable of piercing the skins of animals on the juices of which they feed; others are quite destitute of this power of piercing.
Many feed chiefly on saccharine and other vegetable juices. In some genera, the per fect insect seems destitute of a mouth; and the term of life, after the perfect state has been attained, very brief in some, appears to be brief in all. The power which many dipterous insects possess of walking even on very smooth surfaces, in any position, even with the back downwards, familiar to every one in the example of the common house-fly, has not yet received a sufficient explanation. The opinion that their feet are furnished with disks for the formation of a vacuum, has been called in question, but nothing satisfactory has been substituted for it. The terminal rings of the abdomen in the females of many species, form an ovipositor capable of piercing the substances in which the eggs are to be laid, and composed of pieces which may be exserted or retracted into one another like those' of a telescope. The eggs are very generally deposited in putrescent animal substances, but those of some kinds in the bodies of living animals, some in vegetable substances; the larvm of some live in water; the eggs of a few are hatched within their own bodies, and the larva; of some even remain there till they pass into the pupa state. All the D. undergo a complete metamorphosis. Their hum are destitute of true feet, although some of them have organs which serve for the same purpose; sonic have a distinct head; but others have the head soft and changeable in form, capable of being retracted into the body, and .distinguishable only by its position, and by the organs of the mouth. Those which dwell in water or in fluid putrescent matters, have a retractile tail-like prolongation of the body, terminated by a radiated expansion, which communicates with air-tubes, and constitutes part of a very remark able respiratory system. The larvm of some D. spin cocoons when about to pass into the pupa state: but in others, the skin of the larva hardens and encases the pupa; the perfect insect making its escape by forcing off with its head the end of its pupa case.