DIPTERA, the term used in zoological classification for the two-winged or true flies which form one of the largest orders of insects. Their chief character is expressed in the name of the order (Gr. his double, and rrepa wing) and, with certain aberrant exceptions, flies possess a single pair of membranous wings cor responding with the anterior pair in other winged insects : hind wings are absent and are represented by a pair of small knobbed organs termed halteres or balancers. The mouth parts are always adapted for sucking and sometimes for piercing also, and the various organs combined form a proboscis. Flies undergo com plete metamorphosis and their larvae are always devoid of legs and most often have a much reduced head: the pupae are either free or enclosed in a hardened shell or puparium. The foregoing definition embraces over 5o,000 described species and, of this number, about 3,00o kinds inhabit the British Isles, but new species are continually being found in almost all countries. As a rule, flies are of small or moderate size : some species are even minute, measuring one millimetre long, while some Australian robber flies exceed three inches in wing-expanse with a body length of one and three-quarters inches. The majority of flies are diur nal and frequent flowers for their nectar or haunt decaying or ganic matter of diverse kinds. Some species, however, are pre daceous upon smaller insects, while a number of others, including mosquitoes and horse flies, have acquired blood-sucking habits, chiefly in the female. It is in virtue of this latter propensity that the order has acquired great significance in relation to medicine and public health. The pathogenic organisms of malaria, sleeping sickness, elephantiasis, yellow fever and other diseases are trans mitted to man through the intermediary of blood-sucking flies. Diptera are generally of sombre coloration, but some species (hover flies) are conspicuously banded, others (the green-bottles, etc.) are metallic green or blue, while several kinds are densely hairy and coloured like bees. The sexes are generally closely alike, but in some flies with long antennae those organs are densely plu mose in the males, and in many species the compound eyes are placed much closer together in the male than in the female, some times nearly meeting. Linnaeus first used the term "diptera" in its modern sense.
General Structure.—The head is generally a somewhat spherical capsule, with much of its surface often occupied by the compound eyes and the antennae exhibit diverse forms (fig. 1) of important significance in classifi cation. The mouth-parts (fig. 2, 3) are highly modified for suck ing : mandibles are only present in those flies that feed by piercing, and are lancet-like: the maxillae in such flies are of similar form but usually they are reduced, partly fused with the head and chiefly represented by their palpi : the labium is membranous and forms the greater part of the proboscis, and its apex is ex panded to form two sucking lobes or labella. The thorax is fused into a single mass chiefly formed by the large mesothorax, and the legs have five-jointed tarsi. The membranous wings (fig. 9) frequently have the venation reduced and there are but few cross-veins: in certain parasitic and other flies wings are absent. In the female the abdomen often has its terminal seg ments tubular and retractile, forming an ovipositor. The tracheae are frequently expanded to form large air-sacs, while the digestive system usually has a special food-reservoir opening into it by a narrow duct. In many flies the ganglia of the ventral chain are fused into a single mass and, in some cases, the female reproductive organs are adapted to retain the larvae for a variable period after developing from the eggs.
Classification.—The classification of flies is of a very technical character and is somewhat simplified in the following scheme. There are two main sub-orders which embrace a large number of families, only the more important of which are noted below.