Diptera is a large insect order containing about 90,000 described species. They are generally characterized by the single pair of membranous wings and the halteres, which are homologous to the hind wings in other groups of winged insects. In spite of a few wingless flies this character allows easy recognition of the order in most cases.
Although the halteres can be viewed as atrophied hind wings, they have an important function for the fly's flight. The halteres, during flight operating as oscillating pendula, serve as sensual organ. By the position of the oscillating halteres, the fly/midge can detect and control wanted and unwanted rotation movements of the body.
Diptera are holometabolic insects, the larva are always legless and normally soft skinned. As in other holometabolic insect groups the larva is the major growing and feeding stage whereas the adults are mainly responsible for dispersion and reproduction. They are often brisk flyers, which can cover wide distances by both passive wind transport wind and by active flying. Flies and midges are often associated with temporarily and patchily distributed food resources, such as carcasses, excrements of vertebrates, rotten plants and waters. However, many are also soil dwellers. Accordingly, the larval Diptera are particularly often adapted to wet and temporary conditions and feed on decaying organic matter and bacteria. That becomes evident by the often extremely short development times of dipteran larvae and their soft-bodied, worm-like appearance. In the group of higher flies (Cyclorrhapha) the modification of larval mouthparts with respect to the feeding environment is particularly obvious.
Most imaginal Diptera have sucking and licking mouthparts. Besides to the well-known biting flies and mosquitoes they feed on various kinds of fluids or soluble substances on surfaces, e.g. nectar, rotten fruits or honeydew of aphids and other plant sucking Hemiptera. Bacteria ingested together with the fluids might also play an important role in the diet of adult Diptera.
However, in spite of these generalizations diverse taxa or single species of flies and midges acquired a wide variety of food sources and feeding environments. Many taxa are associated with plants, the larvae feed mainly endophytic (e.g. Cecidomyiidae, Chloropidae, Agromyzidae, Tephritidae, Anthomyiidae), others are parasitoids (among others Bombyliidae, Pipunculidae, Sciomyzidae and Tachinidae) and parasites (e.g. Oestridae, and Rhinophoridae). Predation occurs for example in many larval Syrphidae and in larval and adult Asilidae. Most of these alternative lifestyles and the in prevailing saprophagous lifestyles have in common that the larvae almost exclusively prefer concealed feeding habits. Morphological characters suggest that most of these unusual feeding habits have evolved from saprophagy.
The following brief overview outlines some of the major subgroups recognized in Diptera:
This group comprises the midges. They are a paraphyletic assemblage of rather primitive groups of Diptera. The Nematocera can be diagnosed by the elongated, multisegmented antennae.
Flies. The remaining part of Diptera characterized by reduced number of antennal segments to three.
Cyclorrhapha (= Muscomorpha sensu McAlpine, 1989, not Yeates and Wiegmann, 1999 and Woodley, 1989)
This clearly monophyletic fly taxon (higher Diptera) was erected because of the emergence of the so-called puparium in this group.
Cyclorrhapha is a successful subgroup of Brachycera containing some of the best-known and economically important families as Syrphidae, Drosophilidae, Tephritidae, Muscidae and Agromyzidae.
Some of the most outstanding features that characterize the group come from immature instars: The pupa is surrounded by the hardened cuticle of the last larval instar making up a capsule integument of the soft-skinned pupa. It can be interpreted as an efficient means to survive long unsuitable time periods, especially those that are too dry for larval development. The emerging imagines escape from the puparium via preformed cleavage lines. A further character of the Cyclorrhapha is the peculiar type of larval mouthparts, the so-called cephalopharyngeal skeleton (see Immature stages). The widely accepted view is that the cephalopharyngeal skeleton including a filter mechanism is part of the ground plan of Cyclorrhapha (McAlpine, 1989). This kind of particle feeding is until now widespread across the group. Later in the evolution the function changed in several lineages and the cephalopharyngeal skeleton was modified to enable the larvae to conquer new food sources, as living plants or other insects.
Further characters are the reduction of the number of larval instars to three and the mobile proboscis of the adult flies, which is capable of being retracted into the head capsule.
This is the largest monophyletic subgroup of the Cyclorrhapha. Except the hoverflies (Syrphidae) the best-known higher Dipteran families belong in this group. Of these house flies (Muscidae), Blowflies (Calliphoridae), fruit flies (Tephritidae), vinegar flies (Drosophilidae) and also Family Agromyzidae. The monophyly is supported by the occurence of the ptilinum, an adaptation for emerging from the puparium and for escaping from the puparial locality (see: External adult morphology).