DIPTERA (Aristotle, from 8t, double, and 7rrepa, wings), an Order of the Inseeta, containing the " flies," properly so called, with which, also, in spite of not possessing its chief characteristic, the sub-order Aphaniptera (fleas), a part of the obsolete Aptera, is now incorporated. The Diptera proper (with the exception of the apterous Nycteribiidce, and a few aberrant species of other families, to which the majority of the characters given will not strictly apply, but which cannot, from their general structure, metamorphoses, habits, or evident natural affinities, be separated from the Order under consideration) have the following characters : - wings two, mesothoracic, membrarous, mostly horizontal and transparent, not capable of being folded, with nervures generally few and longitudinally disposed, and having a pair of alulets at the base ; metathoracic wings replaced by a pair of halteres or balancers ; mouth antliate (whence the Fabrician name Antliata for the Order), with a proboscis formed of the labium, inclosing modifications of other usual parts of the mouth, except of the labial palpi, which are wanting ; tarsi 5-jointed ; prothorax reduced to a very small collar. They are divided into two sections - the OaTuoRuamia, in which the pupa is incomplete (the details of the future perfect insect being visible), and the CYCLORHAPHA, ill which the pupa is coarctate (of a hard, uniform surface, cylindrical, rounded at the extremities). The ORTHORHAPHA are again divided into two sub-sections - the Nematocera (antennm composed of more than 6 joints, palpi 4- or 5-jointed), and the Brachycera (antennm short, with apparently only three distinct joints, palpi 1- or 2-jointed). Of tbese, the Nematocera comprise three tribes, viz. :--1, the Oligoneura, in which the wings have very few nerves (fam. Cecidonzyilda.); 2, the Encephala, in which the. larva has a distinct head (fams. Myeetophilidce, Bibionida, Rhyphicke, Simuliidce, Chironomida, Culicidce, and Psychodidce); and 3, the Polyneura, in which the wings have many veins (fam. Tipulidce). The Brachycera also comprise three tribes, viz. :-1, the Cyclocera, in which the third joint of the antennee is annulated (divided into two groups - a, Notacantha, fams. Stratiomyiida, -rylophagidce, and Acanthonzerida ; and b, Tanystozna,= funs. Tabanidce and Leptidce); 2, the Ortlumera, in which the antennae are normal (divided into two groups - c, Polutonza,= fams. Therevidce and Scenopinida .; and d, Procephala, = lams. Acrocerida, Bombyliidce, Nemestrinidce, Mydasidce, Asilidce, Enzpida, and Dolichopodidce) ; and 3, the Aeroptera, in which the wings are pointed (fat% Lonehopteridce). The CYCLORHAPHA in like manner are divided into two sub-sections - the Proboscidea, possessing a proboscis, and the larvm having an cesopliageal frame ; and the Eproboseidea (also variously termed Coriacea or Pupipara), in which the proboscis is wanting, the body leathery, and the larvm have no (esophageal frame. Of these, the Proboseidea comprise three tribes, viz. :-1, the Hypocera, in which the antenme are inserted quite close to the mouth (fam. Phoridce) ; 2, the Pseud,oneura, in which the wings have a false longitudinal veinlet Syrphidce) ; and 3, the Eumyiidce, or type flies (fams. Pipunculidce, Platypezidce, Conopidce, Muscida, and Wstridce). The .E'proboscidea comprise three families, all parasitic, - the Dippoboseida, Nucteriblidce, and Streblidce, the latter a very limited and aberrant group stated to be oviparous, and having the wings distinct and well veined, unlike the Nyeteribiidce.

The sub-order Aphazziptera consists of two families only, the Pulicidce and Platypsyllidce (the latter so peculiar in structure as to have been claimed for the Coleoptera). Its members are parasitic, entirely coriaceous, much compressed or flattened, and destitute of wings or balancers, these organs being represented by more or less obsolete leathery plates ; they have 3- or 1-jointed antennw, 4-jointed maxillary and 3-jointed labial palpi, and 5-jointed tarsi. The larvie of such of them as are known are vermiform, and the ppm inactive, incomplete. If not considered as a sub-order, these two families would apparently have to .be placed at the head of the Eucephalous Nematocera, - in that case, of course, deranging the characters given for that tribe.

Other families of the Diptera have been proposed, but need not be here noticed, being as yet scarcely established, or merely entitled to the rank of sub-families (the Muscidw especially comprising many -of the latter).

The Diptera, in number of species and individuals (very many having swarming propensities), have been considered to be the order of animated beings most diffused over the globe ; and the extremes of heat and cold seem alike indifferent to them. They have long been known to abound in very high latitudes ; and, among the insects brought back by Captain Feilden, the naturalist attached to Sir George Nares's Arctic expedition, were Dipterous species of apparently the most feeble organism. The liev. A. E. Eaton, attached as naturalist to the late " Transit of Venus " expedition, discovered also, on the desolate slimes of Kerguelen's Island, Diptera of a degraded type suitable to the climatic peculiarities of the locality. Tropical countries naturally furnish the most developed and in spine cases extraordinary forrns, - the genera Pangonia, Rhopalomera, Achias, Diopsis, and Elaphoznyia, and various Acrocerida (even in temperate regions) abounding in instances of exaggerated and apparently unnatural struc-ture. To a geographical distribution of the widest extent, the flies add a range of habits of the most diversified nature ; they are both animal and vegetable feeders, au enormous number of their species acting as scavengers in consuming putrescent or decomposing matter of both. kinds. The phytophagous species are attached to various parts of the plant, dead or alive ; and the carnivorous in like manner feed on dead or living flesh, many being parasitic on living animals of various classes (even Reptilia, as a fly is parasitic upon frogs in Australia), and more especially upon other insects, including Hymenoptera, of which they frequently simulate the external facies. No reasonable approximation can be mane to the number of existing species, as the Diptera are not collected or examined with the same assiduity as the more attractive orders. Schiner, however, in 1868, stated the number their recorded to be no less than 20,800, to which a considerable animal increase is being made (e.g., 550 species in 1869, and 230 in 1875) ; and more than 407)0 different genera have been found necessary for their reception. These must be nevertheless taken as vastly below the mark of existent species. No catalogue of the British species has recently been made ; Westwood, - in 1840, enumerated about 2350.

Considered in relation to man, there would seem to bo sufficient reason for placing this apparently feeble order at the head of our insect enemies. Allowing for the good effected by the clearing away of animal and vegetable impurities by many species, and for the indirect advantage caused by the known instances of a, few others assisting in the fecundation of plants, there remains a long list of direct injuries effected by Diptera. Without laying undue stress upon the formation of galls and other vegeta,ble deteriorations caused by many species, there can be no doubt that the destruction of grass-lands by the larva of the crane-fly, " Daddy Long-Legs " (Tipula oleracea), of olive-crops by Dacus, of oranges by Ceratitis, of various culinary plants by Psila, Tephritis, Anthonzyia, Phytoznyza, Drosophila, &c., and of wheat and other crops by the " Hessian fly," Oschtis, and Chlorops, are of very serious consequence. Our domestic animals, moreover; suffer from the bot-flies ((Estrus, Gasterophilus, and Cephenemyia), the tick (lifelophagus)-, gad-flies ( Tabanus, Thematopota, Chrysops, and Stomoxy.s, many of which attack raan him-self), and last, and Most dreaded, the African " Zimb," or " Tsetse," Glossina morsitaus, which is of sufficient power to close the exploration of a, region in which it occurs. Nor is man himself spared ; the petty inc,on-veniences of wasted food, broken rest, and slight personal pain or irritation experienced in temperate regions from fly-larva, gnats, midges, and the parasitic species, are aggravated in both warmer and more boreal countries to a dangerous extent, and have even been found prejudicial to life. There are many recorded instances of the larvm of Diptera feeding upon the human intestinal canal, and of species (dubiously referred to CEstrus) attacking man ; as also of loathsome cases of individuals being eaten alive by the larvm of flies, developed in food secreted about the persons of beggars. Various cases have, moreover, recently been noted of the diffusion of the germs of disease by flies ; and instances of death front transference of putrid animal matter in New Caledonia have also been recorded. One of the 3fuscidx, Lucilia kominivorax, is known to have caused considerable destruction to human life ainong French convicts in Cayenne, laying its eggs in the mouth or nostrils during sleep ; arid a very precise account of much disease and death in man and domestic animals at Mobilew, by a similar action of another of the same family, Sarcophda wohlfarti, has recently been given by Portchinsky, a Russian naturalist. It is perhaps super-fluous to speak of mosquitoes, too well known since the " plague of flies ;" but it may bc observed that the corresponding plague of sand-flies, Simulium, so well known to affect the eyes of sufferers from ophthalmia in _Ezypt, has made its appearance iu the deserts of West Australia, where the last exploring expedition of Mr Ernest Giles suffered severely from it.

The antiquity of the fly is scarcely more than historical. Very few fossil species are known (5 only being recorded from the Solenhofen lithographic Oolite) ; but the more recent " flies in anaber " are so constantly found that the expression has become a conmion proverb. (E. c. n.)