Microdon are found in ants' nests, either feeding on debris within the nest as scavengers or preying on the ant larvae. They are very peculiar looking with dome shaped bodies and an apparent lack of segmentation; in fact one of them was initially described as a new species of mollusc some years ago. Some other genera, such as Eristalis, have another type of larva, known as the rat-tailed maggot, which feeds among decaying matter and sediment at the bottom of pools of stagnant water. While feeding, they raise their long, posterior respiratory tube vertically to the surface in order to breathe. Often all that can be seen of them is a group of breathing tubes sticking upwards from the mud and rotting leaves at the bottom of stagnant pools.

Some hover-flies mimic other species. Volucella bombylans, one of the better mimics, comes in different colour forms that mimic different species of bumble-bee, and it breeds in the nests of social wasps. The adults enter the wasps' nest to lay their eggs, and secrete a chemical that prevents the wasps from attacking them. In the nest, the larvae develop either as scavengers or they may attack the larvae or pupae of the wasps. Once fully-grown the larvae often leave the nest and are sometimes found wandering around houses, when there is a wasps' nest in the attic for example. The presence of large grey maggots crawling around can cause alarm, but they are not harmful in any way.

Tachinids have a number of breeding strategies. Some lay their eggs directly onto the hosts, others lay their eggs onto their hosts' food plant which may be ingested by the host, or in some other cases the larvae hatch on the hosts' food plant and actively seek out the hosts. The larvae, once inside their host, initially feed on body fluids and fat reserves and don't damage vital organs until they are ready to pupate. They typically leave the hosts once fully grown and pupate in the ground, although some pupate within the skin of the host larva or pupa.

The Flesh-flies (family Sarcophagidae) are either parasitic or saprophagous during the larval stage, the saprophagous forms often breeding in carrion. Sarcophaga is a very large genus of fly whose members all look very similar, being silvery grey with black markings. The abdomen has a distinctive chequered pattern which shifts according to the angle of light. Their biology is variable; although their name seems to suggest they are flesh-eating, a lot of them are genuine parasites of other invertebrates such as snails or other insects. Some have even been suggested as biological control agents of snails, where these occur in large numbers. The closely related genus Blaesoxipha is an internal parasite of Orthoptera (e.g. grasshoppers and locusts) during its larval stage. All flesh-flies deposit first instar larvae rather than eggs; Blaesoxipha deposits its larvae on its adult hosts and the larvae then develop internally.

Wohlfahrtia magnifica is another species of flesh-fly, belonging to a different subfamily. It differs from Sarcophaga by its striking abdominal pattern with spots and stripes. Wohlfahrtia magnifica larvae are obligate flesh-eating parasites of mammals, and the larvae will enter wounds and burrow into healthy flesh. They can be a problem in cattle and sheep agriculture in southern Europe, north Africa and central Asia. They are attracted to wounds on newly born calves and lambs, especially where the umbilical cord is broken; flies regularly deposit their larvae at the navel, and the larvae then enter via the wound.

Blow-flies, (family Calliphoridae), tend to be predominantly saprophagous in the larval stage, but a few are parasitic. The adult flies feed by a method which is common amongst the Cyclorrhapha. Most can only ingest liquids, so they regurgitate their stomach contents onto food to liquefy it before ingestion. Calliphora vicina, the common blue-bottle, is one of the most important species in the carrion fauna, and in Britain is usually the first species to oviposit on dead bodies. It is used as a forensic indicator because its rate of development at different temperatures is well documented, and it deposits its eggs shortly after death allowing the time of death to be calculated quite accurately.

Lucilia sericata, a species of green-bottle, is another form of blowfly that also has carrion feeding larvae, but it tends to favour sunnier situations. They are agricultural pests that cause a disease called sheep strike. This occurs when the skin on sheep whose hindquarters are soiled by dung and urine becomes inflamed and ulcerated. The sores attract the flies which lay their eggs, allowing the larvae to develop in the wounds. As the larvae develop, more flies are attracted to lay their eggs, so the larvae can amass huge numbers and the animal's health deteriorates as the wound becomes increasingly infected. Ironically, if only small numbers of maggots are introduced into the wound, they can actually accelerate the healing process with some of their secretions.

Another Lucilia species, Lucilia bufonivora, has a very different life history. It lays its eggs on toads, usually on the front of the head around the nasal cavities. The larvae then burrow their way into the flesh of the toad and eventually kill it, continuing to feed on the flesh after the host animal has died.

The New World Screw-worm, Cochliomyia hominivorax, has a blue green colour and differs from the Old World blow-flies by having three dark stripes on the thorax. It lays its eggs on all types of wound, from a tick bite to anything larger. The larvae then burrow their way into healthy flesh and, as with sheep strike, the health of the host animal suffers as more screw-worms laying their eggs on the wound. A downward spiral is set in motion until eventually the host animal dies. Screw-worms used to be a serious pest in North America, but have been eliminated by the mass release of sterile males so they are now confined to central and southern America. They were briefly introduced into North Africa in 1988 and became established, presumably because livestock imported from South America carried the larvae, and this outbreak had to be eliminated by the mass release of sterile males. The screw-worm maggot gets its name from the bands of spines around its body which look rather like the thread of a screw, and the shape of the larva.

Some blowfly larvae are bloodsucking. Auchmeromyia senegalensis larvae will only attack people who are sleeping on the ground. They lurk among debris during the daytime, and come crawling out at night to attack people while they sleep. Fortunately they can't climb bedposts so can be very easily foiled by sleeping on a raised bed.

Some other blowfly groups also have bloodsucking larvae. Those of the Protocalliphora genus develop in birds' nests and suck the blood of nestling birds.

Infestations of mammals by fly larvae are called myiasis. One African species which can cause this in humans is Cordylobia anthropophaga. The flies lay their eggs either on the ground or on clothes. They are stimulated to oviposit by the smell of decaying body fluids including urine and sweat. They sometimes oviposit on clothes left hanging to dry outside. Eventually, when the larvae are ready, they will be stimulated to hatch by body heat, and will then crawl into the skin.

The family Mystacinobiidae are a very odd group associated with bats known only from New Zealand; both the adults and larvae develop in bat dung and the adults are flightless. Only one species is known at present.

Larvae of Cuterebridae form warbles--a swelling in the host animal's skin that forms around the larva on mammals, mainly in lagomorphs (rabbits and their close relatives) and in rodents. They lay their eggs close to where the animals live. The eggs are eventually stimulated to hatch by the body warmth of their hosts.

Dermatobia hominis is slightly different from other members of the Cuterebridae in terms of its development. It lays its eggs on biting insects such as mosquitoes; when the eggs are ready and the mosquitoes bite a host, the eggs are stimulated to hatch by the host's body warmth. They then burrow into the skin through the mosquito bite.

Dermatobia hominis causes infestations in humans. Buried in the skin, the larva's posterior spiracles reach to the surface, enabling it to breathe. The larva develops in a swelling (or warble) that forms around it and eventually leaves the body when fully grown. They have strong spines on the segments and can cause quite a lot of pain when inside the body. Mosquito bites infected by Dermatobia may also become infested by screw-worm larvae, leaving the unfortunate victim with a double infestation of fly larvae.

Oestrus ovis, the aptly-named Sheep Nostril Fly (family Oestridae), deposits its larvae around the nostrils of sheep. The larvae crawl up inside, attaching themselves to the mucous membrane and remaining there until fully grown. They are then sneezed out by the sheep and pupate in the ground. They also infest humans to a limited extent and may deposit larvae around the nostrils and eyes although they usually don't stay alive for more than a few hours. There is a documented case of Oestrus ovis larvae completing their development inside a person suffering from AIDS. It is likely that this happened because the immune system, which would usually prevent the larvae developing fully, was suppressed.

The Hyperdermatinae, a subfamily of Oestridae, are warble-flies. They oviposit on cattle or deer and the larvae then burrow into the skin. Eventually they end up around the back of the animal where they settle, with a 'warble' forming around them. They remain there until fully developed, then leave the warble to pupate.

Adult Stomach-bot flies (family Gasterophilidae), lay their eggs on horses or closely related animals, either on the flanks where the host may lick them off and ingest them by swallowing, or near the mouth into which the larvae will migrate and eventually be swallowed. They then attach themselves to the gut walls, usually in the stomach or the small intestines, but in the case of one species, Gasterophilus haemorrhoidalis, to the wall of the rectum. They release their grip when ready to pupate and are passed from the body with the dung. Some tropical stomach bot flies include genera that attack elephants or rhinoceroses.

Glossina, the tsetse fly genus, constitutes the whole of the small family Glossinidae, containing only 22 species. Some of them bite humans and transmit pathenogenic single-celled organisms called trypanosomes which cause sleeping sickness. They also bite and transmit trypanosomes to cattle causing a similar disease called nagana. Tsetse-flies are known from central and southern Africa, but there is a fossil record from North America indicating that they were more widely distributed in the past. The tsetse-fly larva is retained inside the parent female's abdomen until fully grown, when it is deposited. As soon as the female deposits the larva, it seeks some loose soil or leaf-litter in which to burrow and pupate.

The Sheep Ked, Melophagus ovinus (family Hippoboscidae), is a wingless ectoparasite found on sheep. It is blood sucking and deposits its fully developed larvae among the wool of the sheep where they immediately pupate. A heavy infestation can sometimes cause anaemia in sheep, but in general they are not a major cause of disease.

This group includes the Dung-flies (family Scathophagidae), so-called although only one genus, Scathophaga, actually feeds on dung. Of this family, by far the most well known species is Scathophaga stercoraria, the males of which form groups of furry yellow flies on cow pats, where they await females to mate with. The females then lay eggs in the dung where the larvae develop. Most other genera of this family are phytophagous (i.e. plant eating): they include leaf-mining and stem-boring larvae but they are not of major economic importance. Adult dung-flies are predatory on other insects, mainly other Diptera.

The family Anthomyiidae are rather drab looking flies, and again they are a mixture of phytophagous and saprophagous forms in the larval stage. Some are of economic importance, one of the most well-known being the Cabbage Root Fly. The larvae of this and some closely related species get into the shoots and germinating seeds of plants, and sometimes damage crops.

The Lesser house-fly, Fannia canicularis (family Fanniidae), is a very common species that tends to breed in chicken dung while other species of Fannia breed in birds' nests or fungi. The larvae of Fannia are dorso-ventrally flattened and spiny and very distinctive. This is to help protect them against other carnivorous Dipterous larvae, such as those of some Muscidae, which might otherwise feed on them.

The Common house-fly, Musca domestica (family Muscidae) breeds in various types of dung or in waste food. It is potentially a vector of pathenogenic organisms but in parts of the world where sanitation standards are high, their threat to human health is much reduced. They tend to cause more problems in areas with exposed raw sewage and where food is not stored in good conditions. Other Muscidae have predatory larvae which are deposited on decaying substances where there are already large quantities of other fly larvae present. Such larvae may sometimes be saprophagous in their early stages and then become predatory later as they approach maturity.

Some adult Muscidae are blood sucking. Stomoxys calcitrans, the Stable Fly, has a sharp proboscis for piercing skin and sucking blood. This species only occasionally bites humans but it can be a significant pest of livestock into which it can transmit some pathogens such as protozoa or possibly parasitic worms. Some Muscidae feed on other sorts of body fluids such as sweat and they can cause quite a nuisance by settling around the eyes or noses of cattle, while on hot humid days they sometimes settle on people.