Fly

• 1. Introduction

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  The fly, a member of the insect order Diptera, can leave the ground in under five milliseconds. Dolichopodid flies of the genus Condylostylus do exactly that when a camera flash goes off. Their order takes its name from a single pair of wings, and it includes more than 150,000 formally described species, with many parts of the world still unstudied. Horse-flies, crane flies, hoverflies and mosquitoes all belong to it. Four questions run through this documentary. What lets such small animals steer so precisely? How did flies get started in a world before flowers? What lives inside the strangest corners of their family tree? And why do people keep returning to flies, as pests, as medicine and as characters in myth?

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• 2. Built For The Air

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  Take the halteres away from a fly and it cannot fly at all. These small knobs are what remain of the hindwings. They oscillate in time with the front wings and work like gyroscopes, sending quick feedback to the muscles that steer. Wing and haltere beat together, but the amplitude of each wing beat can differ, and that difference lets the fly turn sideways. Each wing is wired to two sets of muscle, one for power and one for fine control. Only one other insect order, the Strepsiptera, has a single working pair of wings plus halteres, and it carries them the opposite way round, with halteres on the middle segment of the thorax and flight wings on the rear one.

  Flies tend to travel in a straight line and then snap onto a new one. These turns are called saccades. A typical saccade swings through about 90 degrees in 50 milliseconds. A sight starts it: nerves fire the steering muscles in the thorax, a slight change in wing stroke creates torque, and within four or five wingbeats the halteres trigger a counter-turn.

  In 1927, Charles Townsend estimated by eye that the deer bot fly, Cephenemyia, moved at 600 to 800 miles per hour. The claim was repeated for years, until Irving Langmuir showed it was physically impossible and suggested about 25 miles per hour. Reflexes are quick, but sustained speeds are low.

  The eyes feed all this steering. A fly has a pair of large compound eyes on the sides of its head and, in most species, three small ocelli on top. In some flies each compound eye is split into an upper and a lower region, perhaps to help in swarming. The ocelli register shifts in light intensity, so the fly can react swiftly when an object approaches. Motion-sensitive neurons analyse the flow of the visual scene. One of them, the H1 neuron, tracks horizontal movement across the whole visual field and guides corrections for yaw in midflight.

  Taste and smell are spread across the body. The main olfactory receptors sit on the third antennal segment and on the maxillary palps, while taste receptors line the feet and the wing margins. A fly can therefore taste its meal by walking on it. Receptors at the tip of a female's abdomen judge whether a spot suits egg-laying. Blood-feeding species can sense infrared emissions from their hosts, and many detect the extra carbon dioxide near large animals. Some tachinid flies of the subfamily Ormiinae even have sound receptors, which they use to find the singing bush crickets they parasitise.

  The body plan itself is compact. The thorax has an enlarged middle segment packed with flight muscle, between two segments reduced to collars. Cushion-like pulvilli between the claws on each foot let a fly cling to smooth surfaces. Female horse-flies cut a cross-shaped incision with knife-like mouthparts and lap up the blood that flows. Some flies are such good mimics that careful inspection is needed to tell them from their models. Spilomyia longicornis passes for a vespid wasp, and a few species imitate other insects well enough that only their wings give them away.

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• 3. Deep Origins

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  The earliest known true flies come from the Middle Triassic, around 240 million years ago. That poses a puzzle, because modern flowering plants did not appear until the Cretaceous, roughly 140 million years ago. Early flies therefore had no nectar. One suggestion points to the attraction many modern flies feel toward shiny droplets. Sap-sucking bugs were abundant then and produced honeydew, and fly mouthparts suit softening and lapping up its crusted residues.

  The fossil record suggests three bursts of diversification. About 220 million years ago, many new species of the lower Diptera appeared. Around 180 million years ago, in the Jurassic, many lower Brachycera followed. The third came 66 million years ago, at the start of the Paleogene, among the Schizophora. At the base of the family tree sit the Deuterophlebiidae and the enigmatic Nymphomyiidae.

  Where flies belong among insects has long been debated. Everyone accepted that insects with radical metamorphosis form a single group, and the trouble lay in how its four main orders relate: Lepidoptera, Coleoptera, Hymenoptera and Diptera. Flies are widely placed in the Mecopterida, alongside butterflies and moths, caddisflies, fleas and scorpionflies, and possibly the Strepsiptera. A proposed grouping with fleas and scorpionflies, the Antliophora, has not been confirmed by molecular studies.

  The traditional split inside the order separated the Nematocera from the Brachycera by their antennae. Nematocera have slender bodies and long, many-segmented antennae, like mosquitoes and crane flies. Brachycera are rounder, with short antennae. Later work showed that the Nematocera are paraphyletic, with the Brachycera nested inside groups once placed among them. A 2023 study using mitochondrial DNA reshuffled the Nematocera again, moving the Anisopodidae to sit beside the Brachycera. Not every fly with a wing count of two is simple either, since louse flies of the family Hippoboscidae have lost their wings altogether.

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• 4. From Maggot To Adult

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  A fly passes through four stages: egg, larva, pupa and adult. In many species the larva lives long and the adult briefly, because the adult's only job is to mate and lay eggs. Most larvae grow in sheltered places, such as water, carrion, fruit, fungi, or inside a host. Their thin skin dries out if they are exposed to air.

  Fly larvae have no true jointed legs. Some, including those of the Simuliidae, Tabanidae and Vermileonidae, have prolegs for gripping a surface in flowing water, living tissue or prey. Brachyceran larvae go further. Their heads are soft and often drawn back into the thorax, and their eyes and antennae are reduced or missing. That bareness suits life in carrion or inside a host. They are informally called maggots, though the word is not technical and is often applied to any fly or insect larva.

  Most flies lay eggs, but reproduction takes unusual turns. In the Sarcophagidae, larvae start developing inside the eggs or hatch and mature inside the mother, which suits food that is short-lived. In Hylemya strigosa the larva moults once before hatching. Termitoxenia females have incubation pouches and deposit a fully developed larva that pupates almost at once. The tsetse fly keeps one fertilised egg in the oviduct, where the larva feeds on glandular secretions. When fully grown it is deposited in soft soil, where it buries itself and pupates. Lundstroemia parthenogenetica reproduces without males, and certain gall midges have larvae that produce eggs themselves.

  Pupae come in two kinds. The obtect pupa, common in the Nematocera, shows its future appendages stuck to the body. In the coarctate type, the last larval skin hardens into a puparium that hides the real pupa inside. The adult inflates a balloon-like structure on its head to force its way out.

  Male flies have genitalia rotated from the usual insect position, either temporarily during mating or permanently during the pupal stage. In a full 360-degree torsion, the sperm duct wraps around the gut. During mating the male starts on top of the female, then turns to face the other way. Torsion lets him stay upright. Mating is quick, which helps explain why fly populations grow so large.

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• 5. Niches Everywhere

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  Estimates of species by region run at about 19,000 for Europe, 22,000 for the Nearctic, 20,000 for the Afrotropics, 23,000 for the Oriental region and 19,000 for Australasia. Most species have limited ranges, but the housefly, Musca domestica, is found worldwide. The largest fly is generally thought to be Gauromydas heros, up to 7 cm long. The smallest, Euryplatea nanaknihali, measures 0.4 mm, smaller than a grain of salt.

  Some larvae thrive where almost nothing else can. Those of Megaselia scalaris eat paint and shoe polish. Shore flies and some chironomid midges survive in glaciers, hot springs, geysers, saline pools, sulphur pools and septic tanks. One species, Helaeomyia petrolei, lives in crude oil. Adults of Ephydra hians forage underwater, where hydrophobic hairs trap a bubble of air for them to breathe. Cave-dwelling fungus gnats of the family Mycetophilidae have bioluminescent larvae. Gall midges of the Cecidomyiidae show the greatest variety among gall-forming insects, and the Agromyzidae mine leaves, leaving blisters and tunnels between the surfaces.

  Plenty of flies steal for a living. Satellite flies of the subfamily Miltogramminae trail wasps and take their stung prey, or lay eggs in it. Phoridae, Milichiidae and the genus Bengalia take food carried by ants. Some adult flies regurgitate droplets in a habit called bubbling, possibly to concentrate food or to cool off. Others have turned parasitic. Bot flies of the Oestridae live inside mammals, and the animals parasitised by brachyceran larvae include molluscs, woodlice, millipedes, insects and amphibians.

  Flies are eaten at every stage of life. Birds, bats, frogs, lizards, dragonflies and spiders prey on them, and their eggs and larvae suffer parasites. Their defences include disguise. Many hoverflies resemble bees, wasps or ants, and some tephritid fruit flies resemble spiders. Certain hoverflies live as young inside ant nests, protected by copying the chemical odours of the colony. Not every fly is well defended, however. Drosophila subobscura lacks a type of blood cell found in its relatives and cannot fend off parasitic attack.

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• 6. Friend And Foe

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  Flies rank second only to bees and their relatives as pollinators. In wet and cold places they matter even more. A fly needs less food than a bee because it does not stock a nest for young. Flowers that produce little nectar rely on them, as do flowers that pollinate by trapping visitors. Some plants depend on fungus-feeding flies that visit infected male flowers. Flies may have been among the very first pollinators of any plant. Bee flies and hoverflies pollinate crops today.

  Other flies undo that goodwill. Anopheles mosquitoes carry malaria, filariasis and several arboviruses, while Aedes aegypti carries dengue fever and the Zika virus. Blackflies spread river blindness, and sand flies spread leishmaniasis. Houseflies contaminate food and spread foodborne illness. In tropical regions, eye flies of the Chloropidae seek fluids from the eye, and a few species can cause myiasis in humans. Tsetse flies, screwworm and botflies attack livestock, wounding animals and spreading disease with significant economic harm. Leaf-miner flies, fruit flies and gall midges damage crops.

  Then there is the quieter work of cleanup and pest control. Houseflies, blowflies and fungus gnats scavenge and aid decomposition. Robber flies, tachinids, and dagger and balloon flies prey on or parasitise other insects, which helps keep various pests in check.

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• 7. Laboratory And Clinic

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  The fruit fly Drosophila melanogaster became a standard model organism because it breeds easily, has a small genome, and shares many genes with higher eukaryotes. Genetic studies on it have shaped understanding of gene expression, gene regulation and mutation. Willi Hennig's work on dipteran relationships helped found cladistics, a method he first used on anatomical traits and that researchers now apply to molecular sequences.

  Maggots found on a corpse give forensic entomologists a clock. Species can be identified by their anatomy and by matching DNA. Different flies arrive at fairly well-defined intervals after death, and so do their predators, such as beetles of the family Histeridae. Which species are present, or absent, indicates the time since death and sometimes the place, when a species is confined to a habitat such as woodland.

  In medicine, blow fly larvae are used in maggot debridement therapy. They remove dead tissue from wounds, most often amputations, which promotes cell growth and healing. Their secretions also have antibacterial activity. Battlefields and early hospitals saw the practice for centuries, sometimes by accident. Today medicinal maggots are regarded as a safe and effective treatment for chronic wounds.

  Blowfly larvae, called gentles, and bluebottle larvae, called casters, are bred commercially. Anglers buy them as bait, and keepers of carnivorous pets, zoo animals and research animals buy them as food. Proposals to feed insect larvae to chickens, pigs and fish meet consumer resistance, and feeding insects to animals remains illegal in places such as the European Union.

  On Sardinia, the cheese casu marzu is left open to cheese skippers such as Piophila casei. Their larvae soften the cheese and change its aroma as it ripens. European Union authorities once banned its sale, but the ban was lifted because the cheese is a traditional product made by traditional methods.

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• 8. Flies In Myth And Art

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  In traditional Navajo religion, Big Fly is an important spirit being. Christian demonology gives the insect a darker role in Beelzebub, the "Lord of the Flies" and a god of the Philistines. Flies appear in writing as far back as ancient Sumer, where one poem has a fly help the goddess Inanna when galla demons chase her husband Dumuzid. In Mesopotamian flood stories, floating corpses are compared to flies, and the gods swarm "like flies" around the offering of the hero Utnapishtim. Old Babylonian seals use flies as symbols of Nergal, the god of death, and people in ancient Mesopotamia wore fly-shaped lapis lazuli beads.

  Ancient Egyptians made fly amulets and awarded fly emblems for bravery and tenacity, because a swatted fly keeps returning. Flies may also have been linked to the departing spirit, since they gather near the dead. In some areas of modern Egypt, people still avoid swatting shiny green flies, which may carry the soul of someone recently dead.

  A little-known Greek myth tells of a talkative maiden named Myia, meaning fly. She angered the moon goddess Selene by trying to seduce the sleeping Endymion, and Selene turned her into a fly that still robs people of sleep. In Prometheus Bound, attributed to Aeschylus, a gadfly sent by Hera torments Io, a woman turned into a cow and watched by the hundred-eyed Argus. Io cries out, "Ah! Hah! Again the prick, the stab of gadfly-sting!" Shakespeare echoed the idea. In King Lear, Tom o'Bedlam is driven mad by a fiend who has led him "through fire and through flame." In Antony and Cleopatra, Cleopatra's hasty flight from the battlefield at Actium is likened to a cow chased by a gadfly.

  Painters in the 15th and 16th centuries often slipped a small fly into a picture, a device called Musca depicta, Latin for painted fly. In 1962, the biologist Vincent Dethier published To Know a Fly, introducing general readers to its behaviour and physiology. The phrase fly-on-the-wall describes documentary film-making that watches events candidly. Engineers have drawn on flies to design miniature flying robots. Steven Spielberg's 1993 film Jurassic Park built its plot on DNA surviving in the stomach of a blood-sucking fly fossilised in amber, an idea scientists have since discounted.

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