Most of the species are very small at 1–2 mm.
Dedicated site 
The Acalyptratae or Acalyptrata are a subsection of the Schizophora, which are a section of the order Diptera, the "true flies". In various contexts the Acalyptratae also are referred to informally as the acalyptrate muscoids, or acalyptrates, as opposed to the Calyptratae. All forms of the name refer to the lack of calypters in the members of this subsection of flies. An alternative name, Acalypterae is current, though in minority usage. It was first used by Justin Pierre Marie Macquart in 1835 for a section of his tribe Muscides; he used it to refer to all acalyptrates plus scathophagids and phorids, but excluding Conopidae.
The confusing forms of the names stem from their first usage; Acalyptratae and Acalyptrata actually are adjectival forms in New Latin. They were coined in the mid 19th century in contexts such as "Muscae Calyptratae and Acalyptratae" and "Diptera Acalyptrata", and the forms stuck.The Acalyptratae are a large assemblage, exhibiting very diverse habits, with one notable and perhaps surprising exception: no known acalyptrates are obligate blood-feeders (hematophagous), though blood feeding at various stages of the life history is common throughout other Dipteran sections.Asilomorpha
The Brachyceran infraorder Asilomorpha is a large and diverse group of flies, containing the bulk of the nonmuscoid Brachycera. The larvae of asilomorphs are extremely diverse in habits, as well.Calyptratae
Calyptratae is a subsection of Schizophora in the insect order Diptera, commonly referred to as the calyptrate muscoids (or simply calyptrates). It consists of those flies which possess a calypter that covers the halteres, among which are some of the most familiar of all flies, such as the house fly.
About 18,000 described species are in this group, or about 12% of all the flies yet described.Carnoidea
Carnoidea are a superfamily of Acalyptratae flies.Carnus (fly)
Carnus is a genus of flies (Diptera) with 5 described species, all of which are parasites of birds. The adult flies locate a suitable host nest, then shed their wings and feed on the blood of the developing nestlings. Mature female flies lay their eggs in the nest, where their larvae develop on organic detritus.Carnus hemapterus
Carnus hemapterus is a Dipteran insect, a small-bodied and partly black-coloured carnid fly. In their adult stage of life, they are blood-sucking ectoparasites of nestling birds. Within the genus Carnus, this is the only species widespread across Europe and the cold and temperate regions of Asia and North America. Female body length is about 1.5 mm, males are smaller. It typically occurs in the nests of medium- to large-bodied birds, provided that the nest is not on the ground. It is particularly common on the chicks of owls, falcons, rollers, bee-eaters and starlings. Females give birth to larvae that live within the nest and feed on organic debris and the pupae also overwinter there. The emergence of imagines is synchronized to the hatch of host nestlings in the subsequent year. They prefer larger chicks within the nest. Adult flies have a winged and an unwinged variety, the latter being much commoner. In fact, unwinged flies still carry the basal part of their wings, but the majority of the wing is broken off (see a close view of the above photo). Flies live only on the nestlings before and during the development of the plumage, and disappear later on.Infestations by Carnus hemapterus appear to be rather harmless for the host chicks. In spite of that, barn owl females advertise their resistance genes by the dark spots on their breast and belly; nestlings of heavily spotted females tend to be more resistant. In this study, potential environmental confounding effects were controlled for by cross-fostering chicks.
The faeces and blood remains from these ectoparasitic flies make spots on the eggshell surface of host birds. This contamination appear to increase abundance and diversity of eggshell bacterial loads and, consequently, increase the chance of embryo death.Chironomoidea
The Chironomoidea are a superfamily within the order Diptera, suborder Nematocera, infraorder Culicomorpha. This superfamily contains the families Chironomidae, Ceratopogonidae, Simuliidae, and Thaumaleidae. One of the more important characteristics used to define them is the form of the larval mouthparts.Culicoidea
The Culicoidea are a superfamily within the order Diptera. The following families are included within the Culicoidea:
Dixidae – meniscus midges
Corethrellidae – frog-biting midges
Chaoboridae – phantom midges
Culicidae – mosquitoesCulicomorpha
The Culicomorpha are an infraorder of Nematocera, including mosquitoes, black flies, and several extant and extinct families of insects.Enigmocarnus
Enigmocarnus is a genus of flies (Diptera). There is 1 described species.Hemeromyia
Hemeromyia is a genus of flies (Diptera). There are 8 described species.Meoneura
Meoneura is a genus of carnid flies (Diptera).Meoneurites
Meoneurites is an extinct genus of flies (Diptera). There is 1 described species.Milichiidae
Milichiidae are a family of flies. Most species are very small and dark. Details of their biology have not yet been properly studied, but they are best known as kleptoparasites of predatory invertebrates, and accordingly are commonly known as freeloader flies or jackal flies. However, because of the conditions under which many species breed out, they also are known as filth flies.Neomeoneurites
Neomeoneurites is a genus of flies (Diptera). There are 2 described species.Oestroidea
Oestroidea is a superfamily of Calyptratae including the blow flies, bot flies, flesh flies, and their relatives.The superfamily includes the families:
Mesembrinellidae (formerly included in Calliphoridae)
Rhiniidae (formerly included in Calliphoridae)
Superfamily Tabanoidea are insects in the order Diptera.Tephritoidea
The Tephritoidea are a superfamily of flies. The following families are included:
Pallopteridae — flutter flies
Piophilidae — skippers
Platystomatidae — signal flies
Tephritidae — fruit flies
Ulidiidae (Otitidae) — picture-winged fliesThe Tachiniscinae, formerly ranked as the family Tachiniscidae, are now included in the Tephritidae.Tipulomorpha
The Tipulomorpha are an infraorder of Nematocera, containing the crane flies, a very large group, and allied families.
One recent classification based largely on fossils splits this group into a series of extinct superfamilies (below), and includes members of other infraorders, but this has not gained wide acceptance.
Extant Diptera families