Diptera (δίς, double, πτερά, wings), a term (first employed in its modern sense by Linnaeus, Fauna Suecica, 1st ed., 1746, p. 306) used in zoological classification for one of the Orders into which the Hexapoda, or Insecta, are divided. The relation of the Diptera (two-winged flies, or flies proper) to the other Orders is dealt with under Hexapoda (q.v.).
The chief characteristic of the Diptera is expressed in the name of the Order, since, with the exception of certain aberrant and apterous forms, flies possess but a single pair of membranous wings, which are attached to the meso-thorax. Wing-covers and hind-wings are alike absent, and the latter are represented by a pair of little knobbed organs, the halteres or balancers, which have a controlling and directing function in flight. The other structural characters of the Order may be briefly summarized as:—mouth-parts adapted for piercing and sucking, or for suction alone, and consisting of a proboscis formed of the labium, and enclosing modifications of the other usual parts of the mouth, some of which, however, may be wanting; a thorax fused into a single mass; and legs with five-jointed tarsi. The wings, which are not capable of being folded, are usually transparent, but occasionally pigmented and adorned with coloured spots, blotches or bands; the wing-membrane, though sometimes clothed with minute hairs, seldom bears scales; the wing-veins, which are of great importance in the classification of Diptera, are usually few in number and chiefly longitudinal, there being a marked paucity of cross-veins. In a large number of Diptera an incision in the posterior margin of the wing, near the base, marks off a small lobe, the posterior lobe or alula, while connected with this but situated on the thorax itself there is a pair of membranous scales, or squamae, which when present serve to conceal the halteres. The antennae of Diptera, which are also extremely important in classification, are thread-like in the more primitive families, such as the Tipulidae (daddy-long-legs), where they consist of a considerable number of joints, all of which except the first two, and sometimes also the last two, are similar in shape; in the more specialized families, such as the Tabanidae (horse-flies), Syrphidae (hover-flies) or Muscidae (house-flies, blue-bottles and their allies), the number of antennal joints is greatly reduced by coalescence, so that the antennae appear to consist of only three joints. In these forms, however, the third joint is really a complex, which in many families bears in addition a jointed bristle (arista) or style, representing the terminal joints of the primitive antenna. Although in the case of the majority of Diptera the body is more or less clothed with hair, the hairy covering is usually so short that to the unaided eye the insects appear almost bare; some forms, however, such as the bee-flies (Bombylius) and certain robber-flies (Asilidae) are conspicuously hairy. Bristles are usually present on the legs, and in the case of many families on the body also; those on the head and thorax are of great importance in classification.
Between 40,000 and 50,000 species of Diptera are at present known, but these are only a fraction of those actually in existence. The species recognized as British number some 2700, but to this total additions are constantly being made. As a rule flies are of small or moderate size, and many, such as certain blood-sucking midges of the genus Ceratopogon, are even minute; as extremes of size may be mentioned a common British midge, Ceratopogon varius, the female of which measures only 1¼ millimetre, and the gigantic Mydaidae of Central and South America as well as certain Australian robber-flies, which have a body 1¾ in. long, with a wing-expanse of 3¼ in. In bodily form Diptera present two main types, either, as in the case of the more primitive and generalized families, they are gnat- or midge-like in shape, with slender bodies and long, delicate legs, or else they exhibit a more or less distinct resemblance to the common house-fly, having compact and stoutly built bodies and legs of moderate length. Diptera in general are not remarkable for brilliancy of coloration; as a rule they are dull and inconspicuous in hue, the prevailing body-tints being browns and greys; occasionally, however, more especially in species (Syrphidae) that mimic Hymenoptera, the body is conspicuously banded with yellow; a few are metallic, such as the species of Formosia, found in the islands of the East Indian Archipelago, which are among the most brilliant of all insects. The sexes in Diptera are usually alike, though in a number of families with short antennae the males are distinguished by the fact that their eyes meet together (or nearly so) on the forehead. Metamorphosis in Diptera is complete; the larvae are utterly different from the perfect insects in appearance, and, although varying greatly in outward form, are usually footless grubs; those of the Muscidae are generally known as maggots. The pupa either shows the appendages of the perfect insect, though these are encased in a sheath and adherent to the body, or else it is entirely concealed within the hardened and contracted larval integument, which forms a barrel-shaped protecting capsule or puparium.
Diptera are divided into some sixty families, the exact classification of which has not yet been finally settled. The majority of authors, however, follow Brauer in dividing the order into two sections, Orthorrhapha and Cyclorrhapha, according to the manner in which the pupa-case splits to admit of the escape of the perfect insect. The general characteristics of the pupae in these two sections have already been described.
In the Orthorrhapha, in the pupae of which the appendages of the perfect insect are usually visible, the pupa-case generally splits in a straight line down the back near the cephalic end; in front of this longitudinal cleft there may be a small transverse one, the two together forming a T-shaped fissure. In the Cyclorrhapha on the other hand, in which the actual pupa is concealed within the hardened larval skin, the imago escapes through a circular orifice formed by pushing off or through the head end of the puparium. The Diptera Orthorrhapha include the more primitive and less specialized families such as the Tipulidae (daddy-long-legs), Culicidae (gnats or mosquitoes), Chironomidae (midges), Mycetophilidae (fungus-midges), Tabanidae (horse-flies), Asilidae (robber-flies), &c. The Diptera Cyclorrhapha on the other hand consist of the most highly specialized families, such as the Syrphidae (hover-flies), Oestridae (bot and warble flies), and Muscidae (sensu latiore—the house-fly and its allies, including tsetse-flies, flesh-flies, Tachininae, or flies the larvae of which are internal parasites of caterpillars, &c). It is customary to divide the Orthorrhapha into the two divisions Nematocera and Brachycera, in the former of which the antennae are elongate and in a more or less primitive condition, as described above, while in the latter these organs are short, and, as already explained, apparently composed of only three joints.
Within the divisions named—Orthorrhapha Nematocera, Orthorrhapha Brachycera and Cyclorrhapha—the constituent families are usually grouped into a series of “superfamilies,” distinguished by features of structure or habit. Certain extremely aberrant Diptera, which, in consequence of the adoption of a parasitic mode of life, have undergone great structural modification, are further remarkable for their peculiar mode of reproduction, on account of which the families composing the group are often termed Pupipara. In these forms the pregnant female, instead of laying eggs, as Diptera usually do, or even producing a number of minute living larvae, gives birth at one time but to a single larva, which is retained within the oviduct of the mother until adult, and assumes the pupal state immediately on extrusion. The Pupipara are also termed Eproboscidea (although they actually possess a well-developed and functional proboscis), and by some dipterists the Eproboscidea are regarded as a suborder and contrasted as such with the rest of the Diptera, which are styled the suborder Proboscidea. By other writers Proboscidea and Eproboscidea are treated as primary divisions of the Cyclorrhapha. In reality, however, the families designated Eproboscidea (Hippoboscidae, Braulidae, Nycteribiidae and Streblidae), are not entitled to be considered as constituting either a suborder, or even a main division of the Cyclorrhapha; they are simply Cyclorrhapha much modified owing to parasitism, and in view of the closely similar mode of reproduction in the tsetse-flies the special designation Pupipara should be abandoned. Before leaving the subject of classification it may be noted in passing that in 1906 Professor Lameere, of Brussels, proposed a scheme for the classification of Diptera which as regards both the limits of the families and their grouping into higher categories differs considerably from that in current use.
Little light on the relationship and evolution of the various families of Diptera is afforded by fossil forms, since as a rule the latter are readily referable to existing families. With the exception of a few species from the Solenhofen lithographic Oolite, fossil Diptera belong to the Tertiary Period, during which the members of this order attained a high degree of development. In amber, as proved by the deposits on the shores of the Baltic, the proverbial “fly” is more numerous than any other creatures, and with very few exceptions representatives of all the existing families have been found. The famous Tertiary beds at Florissant, Colorado, have yielded a considerable number of remarkably well-preserved Tipulidae (in which family are included the most primitive of existing Diptera), as also species belonging to other families, such as Mycetophilidae and even Oestridae.
Diptera as an order are probably more widely distributed over the earth’s surface than are the representatives of any similar division of the animal kingdom. Flies seem capable of adapting themselves to extremes of cold equally as well as to those of heat, and species belonging to the order are almost invariably included in the collections brought back by members of Arctic expeditions. Others are met with in the most isolated localities; thus the Rev. A. E. Eaton discovered on the desolate shores of Kerguelen’s Island apterous and semi-apterous Diptera (Tipulidae and Ephydridae) of a degraded type adapted to the climatic peculiarities of the locality. Many bird parasites belonging to the Hippoboscidae have naturally been carried about the world by their hosts, while other species, such as the house-fly, blow-fly and drone-fly, have in like manner been disseminated by human agency. Most families and a large proportion of genera are represented throughout the world, but in some cases (e.g. Glossina—see Tsetse-Fly) the distribution of a genus is limited to a continent. As a rule the general facies as well as dimensions are remarkably uniform throughout a family, so that tropical species often differ little in appearance from those inhabiting temperate regions. Many instances of exaggerated and apparently unnatural structure nevertheless occur, as in the case of the genera Pangonia, Nemestrina, Achias, Diopsis and the family Celyphidae, and, as might be expected, it is chiefly in tropical species that these peculiarities are found. To a geographical distribution of the widest extent, Diptera add a range of habits of the most diversified nature; they are both animal and vegetable feeders, an enormous number of species acting, especially in the larval state, as scavengers in consuming putrescent or decomposing matter of both kinds. The phytophagous species are attached to various parts of plants, dead or alive; and the carnivorous in like manner feed on dead or living flesh, or its products, many larvae being parasitic on living animals of various classes (in Australia the larva of a species of Muscidae is even a parasite of frogs), especially the caterpillars of Lepidoptera, which are destroyed in great numbers by Tachininae. The recent discovery of a bloodsucking maggot, which is found in native huts throughout the greater part of tropical and subtropical Africa, and attacks the inmates when asleep, is of great interest.
It may confidently be asserted that, of insects which directly or indirectly affect the welfare of man, Diptera form the vast majority, and it is a moot point whether the good effected by many species in the rapid clearing away of animal and vegetable impurities, and in keeping other insect enemies in check, counterbalances the evil and annoyance wrought by a large section of the Order. The part played by certain blood-sucking Diptera in the dissemination of disease is now well known (see Mosquito and Tsetse-Fly), and under the term myiasis medical literature includes a lengthy recital of instances of the presence of Dipterous larvae in various parts of the living human body, and the injuries caused thereby. That Diptera of the type of the common house-fly are often in large measure responsible for the spread of such diseases as cholera and enteric fever is undeniable, and as regards blood-sucking forms, in addition to those to which reference has already been made, it is sufficient to mention the vast army of pests constituted by the midges, sand-flies, horse-flies, &c., from the attacks of which domestic animals suffer equally with man, in addition to being frequently infested with the larvae of the bot and warble flies (Gastrophilus, Oestrus and Hypoderma). Lastly, as regards the phytophagous forms, there can be no doubt that the destruction of grass-lands by “leather-jackets” (the larvae of crane-flies, or daddy-long-legs,—Tipula oleracea and T. paludosa), of divers fruits by Ceratitis capitata and species of Dacus, and of wheat and other crops by the Hessian-fly (Mayetiola destructor) and species of Oscinis, Chlorops, &c., is of very serious consequence.
With many writers it is customary to treat the fleas as a sub-order of Diptera, under the title Aphaniptera or Siphonaptera. Since, however, although undoubtedly allied to the Diptera, they must have diverged from the ancestral stem at an early period, before the existing forms of Diptera became so extremely specialized, it seems better to regard the fleas as constituting an independent order (see Flea).
